As a clear example, here's Darth Vader voiced by the actor who played him before James Earl Jones' voice was inserted https://www.youtube.com/watch?v=KQFho0_G1VI

Imagine how different Star Wars would be without Jones.

I'm not sure how you can even watch that scene and not be struck by "oh man, how does this guy have such presence?". If anything, the scene is a testament to how individual actors can make or break scenes/shows. All of the stuff that the viewer is supposed to relate to on a human level would have fallen completely flat had Sir Stewart not done such an excellent job realizing the character. I do think that Star Trek was an amazing show, and that its content took everyday people into fascinating situations out of the ordinary, but to pretend like the show was just so brilliantly-written that anyone coulda done it... definitely not. the show easily coulda been utter dogshit with the exact same script. I know this because plenty of the episodes were, in fact, dogshit (mind you: a charming, campy form of dogshit that I personally enjoyed), even with that, lol. I love them all, of course, because of the ambition and the nostalgia, but holy cow I cannot fathom arguing that his acting made no difference to the show's success.