Shatner owned a small percentage (5%?) of TOS
This is the very first I have ever heard of this. Are you sure?
It's simple. The gang of four spear-carriers started believing all the hot air the fans blew up their skirts. Truth is, Shatner was THE star. Period.
Shatner wasn't an ass, he was just a working actor hustling as best he could to support himself and his family. The man never turned down a paying gig in his life.
That said, he only learned late in life that he had to pander to the Trekies to make them like him, so he did a George Forman and started making fun of himself. He went from a serious actor who took his craft seriously - yes, perhaps too seriously - to a clown.
Personally, I think we may have missed out on getting some great performances out of him.