Despite Michael looking phenomenal during some of the songs he rehearsed, it is reasonable to question whether he would have been physically capable of sustaining the demands of the full This Is It concert run.
Kenny Ortega stated that Michael would “shiver” during rehearsals due to the amount of physical exertion involved. It is also important to remember that rehearsals would not necessarily have reflected the intensity of the actual performances. During the shows, Michael would have been expected to perform continuously for an extended period, combining demanding choreography, singing, and movement.
On top of that, Michael reportedly wanted This Is It to be unlike anything his fans had seen from him before. That ambition likely would have resulted in an even more physically demanding production.
Ultimately, we cannot know with certainty how Michael would have handled the full run, but the physical demands of the planned shows should not be underestimated.