Originally Posted by psylockolussus
Story-wise, its not needed. I don't think the mainstream movie goers would prefer another origin story, another film where Prof X/Magneto are younger, another prequel more than just moving forward in the present.
And "origin" materials can easily be shown in flashbacks ala LOST style.
They still have contracts, Fox won't just let them go, especially with Fassbender and Lawrence being the most on demand actors right now.
Also, if younger versions of Scott and Jean are introduced in the past, these characters will have more dimension next time we see them in a future movie with the original cast.