I'm not really sure why everyone is under the assumption this group will be done after the next film. They're all still young, and, as long as the franchise continues to succeed, there's no reason to think any of them won't want to continue to do it. The only cast-member who might be "too big for Star Trek" is Saldana, but she seems to really enjoy her role as the current queen of sci-fi. As such, I have to think she loves playing one of the pioneering roles for women.