Would have set it back a lot. Eventually we would see it recover but not to same.
Speaking as someone whose earliest memories of serious TV watching was the original airings of ST:TOS, I can say it would have been devastating. Sci-fi suffered terribly in the USA due to the malignant influence of the "suits" (1) in TV corporate leadership. They saw sci-fi on TV as being for kids and weirdos. They had no problem with cheap animation for saturday morning cartoons, but they didn't like the expense of Sci-fi in a weekly television series.
So when they did try Sci-fi, it tended to be in short story format, like the classic Twilight Zone, the not so great Outer Limits, and much more forgettable Science Fiction Theater. The closest you had way back when to a Star Trek was the zero budgeted Captain Video, which to be blunt was just a live action children's show in the same vein as Flash Gordon and Buck Rodgers.
When David Gerrold, the author of the ST:TOS episode "The Trouble with Tribbles", asked CBS about the possibility of doing a live action sci-fi series, the CBS suits told him: "We've already got a science-fiction series. Lost In Space."
Gerrold has had many words to say over the years about that response he was given, none of them kind. Lost In Space's first season, their only BW one, WAS a serious attempt at a sci-fi series, if with a strong lean towards the "Outer Limits-style" episodal writing of "monster of the week". Similar to be honest to the early seasons of Smallville, with it's "kryptonite freak of the week". But after LIS went to COLOR, it metamorphed into a saturday morning cartoonish tale of "A Boy and his Robot", and turning the detestable villain into some kind of ill-defined Big Brother to the Boy. I remember screaming at the TV: "Alright already! When is Major West going to blow Dr. Smith out the airlock in his underwear!?"
For a Star Trek fan like myself, being too young to stay up late enough to watch Star Trek in the 3rd season when it was moved to Fridays at 10PM, the late 60s to the late 70s were a desert.
You could read the novelizations of the series, true. But they were mostly terrible since James Blish apparently didn't know how to write in any format except that of the First Person Main Protagonist. Meaning every time Kirk was unconscious or out of the scene, which wasn't often in a series that I have derisively called "The Kirk & Spock Show", Blish simply skipped the action and showed Kirk learning of the events after the fact, if at all.
After a five year drought, you had the saturday morning cartoon series, which actually wasn't too bad, considering. But the regular non-canon novels didn't start getting churned out until after Star Trek: The Motion Picture.
The sci-fi TV series being produced on TV in the 70s were mostly either fantasy (Wonder Woman) or post-apocalyptic dreck. Special honors however for the Six Million Dollar Man and (esp) the Bionic Woman. Battlestar Galactica of the 1970s? The less said, the better.
TBH, without the push of ST, most TV networks won't take the risk of a space saga pre-CGI, IMO. Though once we reach the 1990s, anything goes.
I keep getting the feeling that I'm forgetting about something huge in the 80s in TV Sci-fi other than STNG, but I can't remember?
1) In fairness to the suits though, Star Trek was always a ratings hole (heavy competition) and very expensive to make. Between ST and the costs of making Mission Impossible, together they ran Desilu Studios into bankruptcy. That's why you see the dancing Orion slave girl at the end of every Season One episode (Desilu), while you see Balok and a starfield at the end of seasons Two and Three (Paramount).