Christopher, I think you and gottacook are talking past each other. The thesis proposed is that we can remove TMP from canon without changing much, not remove it from history.
Well, first of all, that's not a thesis, that's a question. A thesis would be the statement of a proposed answer to that question.
And second,
The Overlord's original question was this:
Star Trek 2, 3, 4 and 6 seemed to have running themes and an ongoing story, which Star Trek 1 and 5 don't really fit into. It seems like you skip those two films because don't really fit with the other 4 films. So are Star Trek 1 and 5 essentially irrelevant?
So the question was about whether their independence from the story arc of the other films made them "irrelevant." Asking whether they can be "removed from canon" without significant impact is a similar but still distinct question. It's certainly not the only valid way of evaluating the films' relevance. In fact, it strikes me as going off on a tangent from the core question.
To address
The Overlord's question purely at face value, leaving aside any of the tangential issues, I would disagree with the implied premise that a story is only relevant if it connects to a larger arc. There is room in an ongoing series for standalone stories as well as arc-based stories. Both can be worthwhile. Every story, even if it is part of an arc, should be strong enough to have value as an individual unit. Does "The City on the Edge of Forever"'s relevance lie in the fact that it got an animated sequel in "Yesteryear," or in the fact that it was a hell of a good story strictly on its own merits? Would it have been less worthwhile if it had never been followed up on?
With all respect, why would a (moderately or otherwise) successful Phase II series have precluded a later Star Trek theatrical film?
It might not have, but the point I'm trying to make is that first there had to be the will on Paramount's part to make ST in movie form at all. It wasn't a given that that would happen. Paramount went back and forth for years between wanting to do a movie and wanting to do a new TV series. So I'm not just talking about the film itself, I'm talking about the
underlying mentality at the studio that
Star Trek was viable as a cinematic entity rather than strictly a television entity. It seems to me that a hypothetical reality in which TMP didn't get made would be a hypothetical reality in which Paramount decided in the end that
Star Trek was meant for television rather than film. Conversely, if Paramount did think that ST was film-worthy at all, it follows that they would've made TMP.
I'll grant the possibility that this hypothetical Paramount regime could've been replaced by a later one that decided to give ST another try on the big screen, but then with different people in charge, they probably would've hired someone other than Harve Bennett to produce the film -- indeed, without the reaction against the huge budget of TMP, they wouldn't have gone to a TV producer like Bennett to save them bucks. So if there had been a movie, it would be impossible to imagine what it might've been like. There are just too many variables.
I was not a fan of The X-Files but I know that two different theatrical films, one during the run of the series and one much later, resulted. There may be other examples.
Actually it was fairly common in the '60s for TV shows to have feature film versions with their original casts while the shows were in production --
McHale's Navy, The Munsters, etc. But it was no longer happening by the '70s, until the Trek franchise came along and proved it viable again -- and even so, it remained unusual ever since, with the
X-Files films not having a lot of company. Paramount was already developing a Trek revival, but they weren't convinced it should be done as a movie until
Star Wars came along and changed everything. But if they'd still, for whatever reason, decided not to turn
Phase II's pilot into TMP as a reaction to
Star Wars, then that would imply it was unlikely they'd ever see ST as a movie property (unless, again, some new regime came in at Paramount). Maybe this is a universe where they decided to develop some other genre property to compete with
Star Wars. Maybe it's a universe where
Star Wars never existed, in which case science fiction might never have come to be seen as blockbuster-movie material at all and would've just stayed in the vein of
Soylent Green and
Silent Running.
Of course there's no way to take this speculation far enough to hazard a guess as to whether Nimoy would have returned in a guest role in Phase II or in any movies (TV or theatrical) that might have resulted; it seems to me that a TWOK-like film could never have existed without Spock.
According to a letter Roddenberry sent to Trek fans on October 22, 1977 (reprinted on pp. 46-7 of
The Making of Star Trek: The Motion Picture), Nimoy "might consider long form television specials" but was unwilling to return to the weekly television grind, since it would detract too much from his stage and screen work.