What the hell are they supposed to do then? If you were forced to do a FC3 and COULD NOT DO AN OT FILM for whatever reason, what are the other options at this point? This is still a "prequel" trilogy that is now in an alternate timeline, just like Star Trek, by all indications. So what concept could they cover with the FC cast exclusively? Another 70's movie? The roster in 1973 isn't very impressive. I am a huge Beast/Magneto guy, but its getting to be a bit overkill with these characters. Gambit on the other hand could desperately use the screen time. As could the other classic X-Men.
It does sound pretty lame that the classic X-Men are going to be tackling Apocalypse as teenagers, but Fox will try to lock that crew down for multiple pictures. However, I don't think they will continue the series with these new actors and not the FC cast, or at least McAvoy's Xavier. I am also not so certain audiences will be keen on X-Men movies exclusively in the 80's and 90's as that wouldn't be driving the concept forward. The X-Men still deal with the same issues of discrimination and genetic profiling in this era, and you can't really cover novel, groundbreaking themes without radically altering the entirety of the X-Men universe and everything it has established in the past 15 years.
Either way it's damned if you do, damned if you don't. As we have seen with Spider-Man, the market isn't going to allow for major X-Men films every couple of years, it's just not. So continuing with a teeny bopper cast in the 80's, Breakfast Club style in new X-films every couple of years won't be financially viable IMO. I would have made Apocalypse a two parter with the FC/OT involved personally, but it doesn't look like Fox has the budget for it.
The ideal solution would be to bump Old Man Logan to May 2016, or late 2016, and then do Apocalypse I/II in 2017 and 2018. Obviously you would need an extra year if Apocalypse is stretched to two films, and that is definitely viable from a business and narrative standpoint. Could have Avengers 3 in May 2018 and Apocalypse II in July as the Marvel summer slate to end all Marvel summer slates. Both the FC/OT must be involved for that scenario to work.