Right, but here's something to think about...
They make it so these movies come out every three years or so. That's how they maintain the rights so Marvel will never get them back.
Although the OT cast has many amazing actors and actresses that the general audience, and us geeks, love, it is the First Class cast that is hotter right now.
Look, we all love Stewart, McKellan, Halle Berry (somewhat), Famke Janssen, Anna Paquin (though she's never been given a good character to portray), Ellen Page and SHawn Ashmore. And I'm sure you'll notice there are some I left off...
But, lets be honest with ourselves. Right now McAvoy, Fassbender, Lawrence and Hoult are way more popular with the general audience, especially the age group that goes to these movies. Add in the fact that Jackman's Wolverine is effectively ageless, so he can be in these, and now the very popular Evan Peters and his amazing portrayal; and a fair question should be asked of why they would ever use the OT cast again.
Any movie they do will focus on these characters, especially Jackman, McAvoy, Fassbender, Hoult and Lawrence. Now think about them creating younger versions of other characters, like Nightcrawler, Ororo, Jean and Scott. And don't forget the concept of a new Gambit. If these characters end up being popular they'll have upwards of 11 characters/actors & actresses that they will continue to focus on. Why wouldn't they?
Which means, after the 2016 Apocalypse movie, they are more likely to use this same cast again, with the story taking place somewhere between 1983 and say the mid-90's. And even if Apocalypse escapes at the end of this one it's unlikely they'll use him again in the next one. By the way, the next one would be in 2019!
For reference, in 2018 when the filming for that movie would happen, the ages of certain actors/actresses will be as follows:
Patrick Stewart: 78
Ian McKellean: 79
Famke Janssen: 54
Kelsey Grammer: 63
So again, it's unlikely.
The one hope that I think we might have is if (and it's a very huge IF) they do Age of Apocalypse for the movie that would come out in 2022.
If they did this, they might be able to use the younger version actors/actresses for Professor X, Magneto, Beast, Nightcrawler, Jean, Scott, Ororo, Mystique and Gambit. They could age them easily enough. Maybe Jackman will still be interested in playing Wolverine, but if not I guess they could (blasphemy) recast him (and any others (Lawrence) who will want out). The newer OT characters of Blink, Warpath, etc. could be used, as well as the OT characters/actors/actresses we all really want of Rogue (done right this time?), Colossus, Kitty and Iceman.
It is possible, but it's unlikely. But if they did get to that point then they could continue in the present with these actors/actresses.