Re: X-Men: Part 6 should be set...
Hugh has showing nothing but loyalty to the Wolverine role. We have no idea if this will be has last outing, when as far as I'm aware, he has said nothing to indicate that it is.
As for your second paragraph, wait "10-15 years" for the FC cast to "grow into their roles a lot better"? Really? That is more likely than further movies with the OT cast? If you make a FC sequel, then stop for the foreseeable future... like anyone will give a damn about the FC cast in 15 years time with no movies inbetween to keep them hyped, to keep momentum.
The series will long since have been rebooted before they "wait" for the FC cast to age.