That last bit has been largely discredited. At least according to Inside Star Trek by Solow & Justman, NBC was highly supportive of TOS and strove to keep it on the air despite its poor ratings (and the attempts of the recent These Are the Voyages books to claim that its ratings were actually high are the result of poor research and interpretation). It brought them prestige, they valued its intelligence, it was Emmy-nominated every year it was on (both for Leonard Nimoy's acting and for the visual effects), and perhaps most importantly, it was cited as the primary reason people in 1967 were buying color TV sets, the patent for which was owned by NBC's parent company RCA. NBC loved ST and was as supportive of the show as it could possibly be. But TV shows succeed or fail based on what the audience wants, not just what the network wants. The ratings were just never all that high, and eventually NBC just couldn't afford to keep making the show any more. But they did bring it back four years later in animated form, and gave Roddenberry complete creative control over the animated series -- which Roddenberry chose not to exercise, instead turning it over to D.C. Fontana and then complaining decades later about how it wasn't "his" Star Trek. And really, every network has had trouble with genre programming. Genre shows have historically been iffy from the start, because they cost more than ordinary shows but tend to have niche audiences and lower ratings, and thus they're at greater risk of cancellation by their very nature. Networks try to do what they can to save their shows from cancellation; after all, it's insane to think they'd go to the trouble of buying a show and paying huge amounts of money to produce it and then actively try to sabotage it. They want their shows to succeed. But there's generally a tension between what makes a profitable show and what makes a good SF/fantasy show, and so many genre shows on many networks have suffered from well-meaning attempts to boost their ratings. And most genre shows get cancelled anyway because they just can't get high enough ratings to justify their expense. This is pretty much true of every commercial network, but the ones that air the most beloved shows tend to earn the most fan resentment -- like NBC post-Star Trek and FOX post-Firefly. They weren't exceptionally harsh in their treatment of the shows, they just happened to make exceptionally beloved shows whose cancellation in the routine course of events was exceptionally painful.