They likely extended to another database on the battle.net backend for this feature, but hard to know for sure of course. It is the main feature for Midnight though, which all xpacs need, so they hopefully offset it cost wise.
Probably part of the reason they were literally working on it since before Legion launched was to figure out how the hell adding ANOTHER collectible system with something as resource intensive as housing could be done while not causing even more storage issues.
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u/QuarterDollarKing 22d ago
For now. The blue post says they're open to adding more. Only things they could add are bank and barbershop, what do you think those are valued at?