X-Men: Apocalypse - 15 Questions Left By The Stupidest X-Movie Ever
2. How Did Professor X Lose His Hair Originally?
The First Class trilogy could never quite decide at what rate James McAvoy would become Patrick Stewart, but, after some odd plot turns, he's firmly in the wheelchair and has finally lost his hair.
In the end, Charles went bald during the failed consciousness swap with Apocalypse, probably an unexplained side-effect of him trying to psychically resist. Now that's a fair way for it to happen - it's got a link to his powers and makes the dome something of a scar - although having it at the hands of En Sabah Nur leaves a major plot point open; we still don't know how it happened in the original timeline.
We can safely assume Apocalypse didn't return in the classic trilogy's continuity - there was no 1973 inciting incident to provoke the cult that awoke him, nor any trace of his mass destruction - so there must have been another, equally seismic event that caused Xavier to go bald. Which I guess works, but kinda undermines the whole idea of the prequel enterprise.
In addition, it undermines what Kinberg said with Wolverine about everything working itself out in this timeline. It's almost like they don't understand the continuity...