It’s not like the folks on NCIS or Law & Order SVU get to play evil versions of themselves for an episode, and if they do it’s usually be because the show had a drastic dip in quality not because the episodes required intense imagination, like the sci-fi series did.
Premises like this led to Star Trek’s success and turned it into the massive franchise it is today.
It would be cool to be a part of a drama where completely nonsensical things happen and that’s supposed to be normal, so I can understand William Shatner’s reasoning.