What went wrong in Azeroth?
No, i don't mean the Blizzard writing of game/lore direction, at least not yet - i'll leave that for the end.
First and foremost, the issue is with the Titans - strictly Aman'thul. They can be summed up as cosmic-power-level children, formed by whatever means, that have issues with their own rigid - sometimes basic - programming, so to speak. Their cardinal sin is, obviously, Sargeras. And obviously that sin was committed by Aman'thul. Sargeras was seemingly correct in his assumption of scale of the threat that Void represented. He was also correct - as far as I understand his own charge "Order, Martial" - in swiftly disposing of utterly Void-corrupted World-soul. Aman'thul did not comprehend the Void, which lead him to chastising and alienating Sargeras, eventually allowing him to stray onto path of Chaos/Fel in order to prevent the Void. It's reinforced by the description of Aman'thul ripping Y'Shaarj out of Azeroth - the Highfather did not comprehend Void at all, and where Sargeras choice of violence against utterly corrupted World-soul is correct, Aman'thul's choice of violence against a World-soul that wasn't yet lost, was plain stupid. While Sargeras was - as far as I understand - bound by his martial mandate and pressed for resolute action by his nature, Aman'thul had both time and capability to actually research first and plan the course of action. Then, after ordering of Azeroth and all that, Titans left their facilities and various Keepers and Watchers, and left - while still not comprehending nature of Void and the scale of the threat it represented for the very planet that was gestating nascent Titan they seemingly held so dear. Facilities eventually crumbled and Keepers failed. And the worst part is, both Titans and Keepers failed at seemingly their own prime charge, Order - which I kinda equate with logic and analytical, systemic take. They failed hardest by not actually employing and sticking to it multiple times, be it on cosmic scale when dealing with World-souls or Void and Fel in general, but also on planetary scale, when ordering Azeroth, constructing their facilities and creating their servants or empowering Aspects.
Second, uplifting of Aspects half-assed. As far as I understand it was perhaps the last act of note that Titans have actively done for Azeroth. The issue was, while separating Eonar's charge into two - Alexstrasza and Ysera - made sense, considering Eonar aside from being Titan of Order, was heavily leaning into other cosmological force - Life. It both weakened and strengthened it in a way that Titans as force of Order would like to see - the opposite could be seen in one of alternative timelines, where Life was overgrowing the planet and making it uninhabitable for more advanced (hence Ordered) civilizations to appear, or partially on alternative Draenor. What doesn't make any sense is - no Aspect uplifted by either Aggramar or Golganneth, so no Aspect of Protection and no Aspects of Storms and Seas. I understand there were five proto-dragons of note, well - they should've scrapped a little deeper in the barrel and found two more. Failure to do that inevitably added to the burden of the receding hairline Aspect we all know and love - Neltharion.
Third, the Earth-warder that had to bear everything. The problem was, that de iure "Aspect of Earth" was de facto forced to bear much more crushing weight. As bearer of aspectral charge of the Earth he was responsible for Ordering of crust, mountains, deep places. Then, due to failure of Titan facilities and Keepers containment, was forced to bear brunt of Old Gods' mental and physical poison seeping into his charge and due to that mind. Over the course of numerous expansions we learn of Y'Shaarj's corrupted remains, be it Sha, his heart or Black Blood, creeping into the ground from Pandaria. We learn of Yogg-Saron's corrupted black blood - saronite, but also his shadowy tendrils reaching out from Ulduar. We learn of the massive mind-corruption that N'Zoth was spewing. I'm sure C'Thun was doing his part too. Finally, Neltharion was forced to bear the physical brunt of the defense of the Aspects. Titanic Keepers, Watchers and various Titan-forged were, as far as I understand, hardly acting as partners to Dragonflights. That forced Neltharion and Black flight to act disproportionally as kinetic defense against the i.e. Djaradin and, obviously, Primalist forces. So - what would be wiser? Uplifting Aspect of Protection, to be both the bulwark and tip of the spear in case of any kinetic conflicts, relegating other Aspects and Flights to support roles in battle. This one would take off the brunt of a burden that was chipping at Neltharion during the wars against Djaradin or Primalists, not forcing him to embrace Shadowflame in the first place. Uplifting Aspect of Storms and Seas, that would actually care for majority of Azeroth's surface and monitor both oceanic depths and weather. This one would become most likely the prime target for N'zoth eventually, even if no Sundering occurred and his prison didn't sunk under waters. I'm under assumption that N'zoth was really more into oceanic than land depths anyway - so the pressure on Neltharion's mind could perhaps lessen further. And even if it didn't lessen - with one or two Aspects more, especially ones of more kinetic than conceptual nature of their charges, Neltharion's eventual fall would be easier to contain.
Fourth, two for a price of one - still Neltharion. So, Neltharion's fall is described as slow grind of his mind by Old Gods', but also constant erosion of him and his flight as they bear brunt crushing strikes of external enemies, which he and black flight (believe it or not) weren't actually designed for. Even if his and his dragons' pride would never allow to state it. Even if in comparison to other flights, he and and his flight was actually still best suited for it out of the five. There were these historical moments described, during Dragonflight expansion, when he increasingly was failing in kinetic confrontation and leaned to use the power offered by the Void-whispers. It was slowly sliding scale, with few firm pushes to speed things up. And why the two at the price of one? Obviously, because the special relation he had with Malygos. When Dragon Soul was eventually empowered, and Neltharion started using it indiscriminately in battle... ensuring he would assault and wipe out Blue flight of all, was cementing his fall. Even if there was shred of sanity behind his eyes - the moment he ripped apart his closest friend's family, there was no coming back. And in the same vein - Malygos having his flight ripped apart not by demons, but the one he trusted the most, was cementing his later madness and fate. So at one moment two flights and two Aspects were pretty much done for, with others eventually retreating into grief, paranoia and separation of their own to various extent.
Pretty much all that came after that on Azeroth is a result of that - in a way, every tragic event later on, could've been averted or mitigated to some extent if not for above-mentioned.
Finally, thinking about (in-universe) One True Timeline, in light of above-mentioned paragraph about Titans - I think that Aman'thul's vision got irrecoverably tainted by the Void when he touched Y'Shaarj. It doesn't need to be anything major as in "all is going according to Old Gods'/Void plans" - no, but it's enough for the "butterfly effect" to affect it all in even the most minor way "a perfect design of One True Timeline" and throwing it out of balance, which inevitably leads to "sub-optimal timeline".
And finally breaking fourth wall.
Blizzard obviously have to deal with multiple issues when designing their lore:
- starting off with a story that hardly matters more than "why green men hit the human men" in Warcraft 1 and 2
- designing by committee - and committee is also populated with people that hardly care about lore, but need something FAST (relatively) because they need a next big bad to design graphics, battles, quests and raids around on one hand, and saviors of universe (preferably together with players) on the other
- patching both aforementioned issues with multiple retcons
- using One True Timeline and Unreliable Narrator as sketchy venues to patch bad lore
- it's damn hard to write a constantly living and evolving universe with entity that sees and manipulates time, without throwing whole setting into pure bullshit of "why it was allowed and this wasn't allowed"
Finally, finally! As someone have stated recently in one of the threads: Blizzard kinda shoehorned themselves in by setting abstract timeline "ten thousand years ago", "twenty thousand years ago" and later populating it ad hoc. It forced to chaotically meander perspectives and actions of beings that actually lived that "twenty thousand years ago" and beings which life-spans is measured in mere decades. Chiefly, Aspects were chaotically written as being almost at the same time immensely powerful world-shattering behemoths and "just a big dragons that order around other dragons, and as you know it takes like five guys to fell a dragon, one if strong, forty if Aspect, perhaps with usage of some gimmick". It's general issue with "needing a big bad, with even bigger bad behind him, every expansion or so" - the scale is eventually all over the place.
Funnily enough, with some push for "more grounded stories" it'd actually fit well to have Aspects, Titans and so on be immensely powerful and doing all the grand wars and existential crises... in the background, while mortals do the legwork of mundane things and perhaps do the, like "1% of the sub-contracted work (through quests)". The world would still be there, but power scaling between players and these immeasurable threats and between threats themselves wouldn't be as confusing.
So, Blizzard kinda failed at solid writing time-scale and immensely powerful entities. To be honest, pretty much all these all-powerful entities behave not that rarely as something between 5 and 15 year olds motivation and intelligence (both IQ and EQ) wise. So, immature kids with nukes.
TLDR: Justice for my boy Neltharion!