But it wouldn't be until the next film that the change really sticks.
Trek movie audiences want and demand a stable status quo. They want the same familiar characters in every movie, and that pressure for familiarity has been a stranglehold on Trek movies for three decades. No real, lasting change in the core cast has ever been allowed while any Trek movie series has been ongoing. I see no reason to believe that's going to change now.
While I agree, we also have to take into account that the TOS cast were very loyal to the franchise and even then, Majel Barrett largely bowed out due to behind the scenes issues, Grace Lee Whitney was considered superfluous, and Nimoy flip-flopped a couple of times. We can't assume that the NuTrek cast will have the same longevity after the 3 movie deal is done, particularly when Pine and Saldana's stars are on the rise. It's harder to tell with the other cast members - Urban and Pegg are genuine fans and would probably be keen to appear if at all possible, the others less so.
Still, I suppose that now they've bitten the bullet once, they could always re-cast characters as and when required if they want to carry on beyond the third movie. Or it may be that the movies make enough money that they can offer salaries to keep all the actors interested.