Originally Posted by Marv
The event that they're trying to prevent takes place in 1973. By going back and preventing said event the timeline changes, eliminating all the events of the original trilogy. Those events now only exist in an alternate reality.
Bryan Singer isn't gonna eliminate all the events of the original trilogy while he already said he's not ignoring any films and that will still be the same case after DOFP ends. Plus the original trilogy and The Wolverine would just look really pointless if they are gonna wipe them out just because of 1 film.