The truth of the matter is that Singer, Hayter, Kinberg, Penn, Dougherty, Harris, Benioff, Woods, Hood & whoever else contributed to the writing for the first 4 films, wrote themselves into a wall. Could an X4 have worked instead of Days of Future Past? Sure. But frankly the possibilities after Days of Future Past seem much better: -Cyclops is alive and can finally lead the team -Xavier & Magneto are in their 50s instead of their 70s (and IMO Fassbender's Magneto is much better than McKellen's) -Mystique has a voice -Jean Grey & Storm are younger & can be put to much better use -Wolverine can still join the X-Men, same goes for Gambit. -Sabertooth doesn't have to turn into a mindless thug Bottom line: the roster & rogues gallery is much better with the timeline being rebooted in Days of Future Past.