Shatner's not Trek. If anything, there has been and will be a lot of Trek that doesn't involve him. But even TOS wasn't Shatner. He did fit like a glove to the character(or vice versa) but as was said his acting left things to be desired.
Also, the triplet Shatner, Nimoy, Kelley was the machine that run the show, they just worked very well together. WASN'T Shatner all by himself. And they did receive help from the other characters as well, I can't honestly think Shatner can complain with his Trek use, he was used the most to the detriment of most other characters and -- since Bond was mentioned often to the point of Kirk becoming a caricature, like Bond became with Moore as the actor incarnating.
After a point, I guess they were fed up with the hassle it needed to bring Kirk back and killed him off. Not likable by some, but good decision since they didn't want to use him anymore. And there is always some complain from him when they don't use him, well, in all truth they don't need too.
He's a very successful actor and author, he doesn't need Trek if he invested his money wisely.
Spock/Nimoy is obviously much easier since Vulcans have huge lifespans and they don't need to invent outrageous theories out of thin air to revive him.(That said his book on the issue was a great way, would be nice to see it performed by the new cast at some point).
Trek shouldn't need any character of the old cast as well, not even Nimoy, but he is the obvious choice, plus he has developed his acting through the years and still remains the best Vulcan portrayal ever, even if only a halfbreed.
Not to mention he's been in Star Trek before Shatner.(Yes, Cage)