Originally Posted by JackMercy
FOX likes to make money, just like any studio. You don't spend the money and the time in logistics of getting all of these major actors together again purely based on a 'story whim.' There are always industrial considerations. And First Class made less than any other X-Men film relative to its cost. Where do you go from there? You hire back and repair relations with the guy that started it all. He gets the original cast back. Boom - you have instant (nearly) guaranteed blockbuster. Pretty simple logic, methinks.
You forget though that Singer is only directing DoFP because Vaughn dropped out. It was not some kind of grand plan on Fox's part to get the guy who started it all to direct again.
And if DoFP was meant to pass the torch back to the OT, they'd be the main focus of the movie. Whereas, if the movie is mainly set in the 70s, it will spend most of the time with the FC cast and Wolverine (who is a big player in the OT, but he's also somewhat separate to the rest of that cast after his own solo movies). Not to say that the studio would not consider another movie with the OT cast, but their appearance in DoFP is not a 100% indication of that. DoFP is the studio's big epic mash-up movie and a bid for Avengers-style success. It's also likely to be a one-off thing, so what they're doing afterwards is probably not clear even to the studio and filmmakers themselves.