I am not disagreeing with you, stock footage doesn't necessarily mean cheap or bad. The Incredible Hulk's first season stock footage episodes were hit or miss though.
I'm not making any value judgments. I'm just saying that, given how much TOS needed to economize to get on the air, it's surprising that they never attempted to build an episode around stock footage, as various other shows did. Just as it's surprising that they didn't follow Roddenberry's suggestions of borrowing sets from just-completed movies or doing multiple episodes set on the same planet or station to amortize the cost of the sets (Starbase 11 being the only time they did that).
Or maybe, they simply didn't think of it or there weren't films with footage to match the stories.
But it would be done the other way around, of course -- they'd find what potentially useful footage they had available,
then build the stories around it.
As I already said, maybe the problem was that Paramount didn't have that many color SF films in its catalog that might have had material adaptable to a Trek episode. Maybe they could've written a planetary-disaster story around FX footage from
Crack in the World, say.
Yup, I read all the same books you did

But that's not what I meant. I meant what you concluded with: Gene himself being the problem. For a show NBC was allegedly proud of (something stated in books decades after fans made it a success and by retired execs not wanting to be villains anymore), they moved it to Fridays for 2 years and in the third season made no attempt to save it.
The show was on the bubble in seasons 1 and 2, but NBC kept it on despite the low ratings that would normally have gotten it cancelled. So hell yes, NBC made an attempt to save it, successfully,
twice. But by season 3, they just couldn't afford to keep it on anymore. Although, don't forget, they
did bring it back in animated form just four years later. They wouldn't have done that if they hated it as much as Roddenberry's myth claimed.
Besides, the show got Emmy nominations three years running, if only for Leonard Nimoy and the VFX. Why wouldn't NBC be proud of that?
Call it a myth, but considering how audiences responded to high promotion (The Man from UNCLE and Batman), investing in a strong ad campaign and putting it in a better slot would support the claim that NBC wasn't going out of their way to keep the show on.
It wasn't in a vacuum. They tried to put it in a good slot in season 3, but George Schlatter insisted that
Laugh-In should have that spot, and
Laugh-In was a hit, so it won. A network has to juggle dozens of shows, and there are going to be winners and losers. So it doesn't make sense to assume the scheduling decision for a given show is exclusively about that show. Sometimes it's just outweighed by other shows.
As for another network having a better relationship, considering how ABC was more open to SF, gave it better time slots and let their producers have at it, they may very well have had a longer run there (NBC probably would have ash canned Voyage after 2 seasons if not 3). But naturally, it's all hindsight and fanciful speculation.
All three of Roddenberry's creations that made it to series,
The Lieutenant and the live-action and animated
Star Treks, were NBC shows. He never successfully sold a series to any other network (since TNG was syndicated). CBS turned down
Genesis II (after having previously rejected ST) and ABC turned down
Planet Earth. NBC passed on
Spectre, but did pick up
The Questor Tapes as a series until Roddenberry himself scuttled the deal. So I'm skeptical of the idea that he would've had a better relationship with a different network. NBC seemed to be the only network that
was willing to do a show with him.
The Incredible Hulk is a textbook example of a show made to appeal to kids and adults.
Age range has nothing to do with the intelligence or sophistication of a show. There are brilliant kids' shows out there, and plenty of idiotic adult shows.
Hulk was more intelligently written than any other '70s-'80s superhero show, the only one of its genre that managed to resist network pressure to dumb down its writing.