Agree the supporting players were supporting players, but there were plenty of "ensemble" shows before the '80s -- Bonanza, Big Valley, Dark Shadows, Mission: Impossible, Beverly Hillbillies, etc. Even Lost in Space started out as one, before it became the Dr. Smith show.
Yes, and the fact series of that kind existed before and during TOS should have been a clear message that on
Star Trek, they were never meant to have a larger role on the same standing as the series leads. Think of this example on the 1966-68
Batman TV series, Stafford Repp (Chief O'Hara) and Madge Blake (Aunt Harriet) were actually listed as main players by contract, reflected in the main credits, yet at no time did they--being main players--ever complain about their status, or believe they were entitled to be treated (as actor or their characters) as Adam West and Burt Ward, the true stars of the series.
Repp and Blake seemed to have a mature, professional understanding of the hierarchy of a series, and which actors were its focus, unlike Takei, Nichols, Doohan and Koenig, whether they were deceiving themselves during TOS' production, or allowed the attention received as the
Star Trek phenomenon became a cultural Mount Everest starting in the early 1970s.
The bottom line of it all, if Shatner had not won the role of Kirk, I doubt anyone else would have put his kind of magnetism/interpretation into the character, played off of Nimoy in such a perfect way, or played a major role with that performance selling the series in the first place. In other words, with anyone else, TOS might have been a mid-season cancellation, or one in a long line of single season shows that year (e.g.,
The Time Tunnel, The Tammy Grimes Show, Love on a Rooftop, etc.). The whiners should have been grateful, but such mature behavior was not to be found in their er...skillset.