Mass re-casting does not require a reboot. I'm pretty confident that modern audiences can cope. I loathe reboots generally, although I give NuBSG a pass.
Not all reboots have to be reimaginings, for example.
A reboot allows us to have the characters without having to deal with the storylines the movies created. You want Rogue as an established X-Men member as opposed to child just discovering them? You can do that. You want Jean Gray to still be alive or Senator Kelly to still be campaigning against mutants, you can do that. You think the Phoenix Saga was poorly done, you can do it again. It allows you to focus on the heart of X-Men stories without having to deal with the baggage the movies created.
Especially since a reboot wouldn't happen until probably 2020 at the earliest, I think they would have earned the ability to start fresh.