Except Singer never did that. Stop trying to imply he would go along with that moronic writing.
He actually reversed many of those events with the timeline reset in DOFP.
How is it moronic to kill off Xavier and Magneto? The comics worked just fine without them, it just means the movies would have to give the other characters who never got much to do...more to do.
The kill-off/depowering list is a wee bit longer: Prof X., Jean, Cykes (made him a complete wuss...), Rogue (when the last two movies sent her on a path to empowerment), Mystique and Magneto. Oh, and Bobby Drake being a cheating bastard. The list goes on... This movie is a series of mis-characterizations one after the other.
Cykes was wasted the entire series, Rogue's decision kind of made sense given how she really wasn't learning how to control her powers, Magneto was ALWAYS the way he was in X3, Mystique...well she got sold out by Mags, etc.
X3 didn't really derail anyone.