Well what if, at the very end of the first Lord of the Rings, you suddenly got this flash forward scene where good guys win, Sauron is defeated and all is good in Middle Earth? Of course I know 100% that this is going to happen, but I don't want to see it cemented before the series' end.
It didn't bother me in First Class to know for sure that Erik, Charles, Hank and Raven are not going to die, but prequels are a completely different proposition - their specific purpose is to tell a backstory to the existing movies. They're not really about "what's going to happen" but "how things happened".
With DoFP, they did the whole time travel story to release the FC cast movies from the constraints of a prequel, but then immediately locked the final outcome down, so it's like these movies are now prequels to a 5-minute sequence.
And frankly that whole sequence was like fanfic brought to life.