Re: How many more movies would you like with >Original Cast<?
I don't think they're going to come anywhere close to having five, if they even do another one with the original cast after DOFP because I could certainly see that happening.
Personally, I think the more feasible idea is just to use DOFP to reset or start a new alternate timeline then just continue on with the First Class cast. That way they aren't bound to any continuity from the earlier franchise so they can recast roles such as Storm, Jean Grey, and Scott while sticking the with up and comers from First Class.
It would really be a shame, and quite frankly, kinda stupid of Fox to ditch actors such as Lawrence, Fassbender, McAvoy, and Hoult who are very much on the come up in new Hollywood right now in favor of the older cast. Not to say that the older cast are inferior but A) they can't keep playing the same characters for 3-4 additional movies, nor would any of them probably want to and B) they don't nearly have the same star power (maybe except Jackman) to pull new people in.
It also keeps the franchise fresh and deters stagnation.