I think ideally Fox would like to have their cake and eat it too. If they think they can make both FC 3 and X4 and make profits from them, they'll make them. I think the difference though is that that a third FC film is almost a sure thing at this point. If Fox has Fassbender, McAvoy and Lawrence on board for a #3, it's as good as made, especially because of Lawrence since -- deserved or not -- she is at the top of the casting lists these days.
I also see no reason to think that bringing the OT cast back was anything other than Singer's idea based on the plot he had in mind. I don't think Fox had any grand plans to return to the OT cast: I think in the wake of X3 they were hoping to create a bunch of Origins-esque spinoffs, but backed off when they realized the only character they had developed enough was Wolverine, at which point the plans for the OT cast were in limbo.
I do, however, think that Fox would like nothing more than to make as many profitable X-films as possible, and I wouldn't be surprised if Singer's storyline has Fox thinking they can find a way to use both casts in the future.
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.
I don't see a traditional
First Class sequel happening beyond this...then again, Marvel's attempting a similar scenario, testing the market to see if 'solo' films work after
The Avengers. I just don't see a
First Class direct sequel having the same draw -- the anticipation's just not there. FOX knows this -- thus
DOFP. They 'test' for these kinds of things, you know...
Is this project partially about giving Singer a chance to do something a little more fulfilling as a climax for his universe of films? Sure, a bit of ego's involved here (isn't it everywhere?) ...But it doesn't mean he'll do exactly what he was going to do with them eight years ago, either.
Bottom line is, if
DOFP 'works' as a good story, general audiences won't care whether it's a 'retcon' or semi-'reboot.' They want to be entertained. And if a filmmaker can do that while simultaneously playing 'meta' with the film series within the narrative itself, it becomes even more meaningful!