It's worth keeping in mind that a given TV show probably only has access to a finite number of stunt performers who have to double multiple people. Sure, no doubt they do their best to hire stunt performers who are roughly the same type as their leads, but a given performer would probably be working on multiple TV shows and might not always be available for a given scene. The top priority is naturally the performer's ability to do the stunts safely, so sometimes physical resemblance has got to be a secondary concern.
So it's just a case where the viewer has to be willing to suspend disbelief. Personally I think audiences today are too spoiled, demanding that everything be totally photorealistic and perfect. For thousands of years, all we had was theater, usually without any set decorations or anything but dialogue to set the scene, and where things like stunts and special effects were always quite clearly artificial. The audience had
to use their own imaginations to bridge the gap between what they were shown and what it was supposed to represent. So if something didn't look like it was real, that wasn't a problem because they had the imagination to make it work. I think the problem with modern special effects and production values is that they've caused the viewers' own imaginations to atrophy.
So the stuntman doesn't look like Shatner? So what? It's an illusion. An illusion that depends on the audience's willingness to participate, to engage their own imaginations and make
it Kirk in their own minds.