Shatner wouldn't have agreed to that.
Agreed. What I think they should do (if there are sequels) is continue with this story and find some way to bring back Shatner. With Old Spock being present, I am sure they can find some way. Heck, if they followed Shatner's books, I wouldn't mind seeing the Borg again if it meant bringing back the original James T. Kirk. (Although I do like the nuKirk

)
It's not that complicated. Now that there's a new timeline, Kirk doesn't necessarily get trapped in the Nexus, so future Kirk from the new timeline can always go back in time.....
But I don't think they should do that. Enough with time travel for now. I hope they move on to something else, and don't try to shoehorn in more of the TOS cast.