Shatner is by no means perfect, but he hasn't spent the last 45 years slagging off his co-stars, he did what most other actors would do, look for more work, Nimoy did the same, Deforest just gave up acting, only coming out of retirement to be in the star trek films.
The three of them were the stars of the show, the others were little more than recurring extras, there for a little colour and diversity, Scotty came off the best because at least he had unique skills, there were plenty of people who could sit at the helm, navigate, and open hailing frequencies.
Before trek they were guest stars, drifting in and out of other peoples shows for one episode at a time, they didn't have great careers before, but suddenly afterwards when they still didn't have great careers this had become somehow Shatner's fault.
I once read a Doohan interview where he said he would have had a career as great as Jack Nicholson, if only he had been given a few more lines, and a few more close ups.
Some of these people need to know there is a difference between "a show was successful, and I was in it" and "a show was successful because I was in it"