Thanks for that.
I've been reading books by TV writers and producers who worked in the 60s and we've been digging through Broadcasting in addition to Variety and it really does change your perspective on the choices the networks made and why, and NBC becomes far less of a villain than fandom has painted it for the past half century. Not to say they always made the right decision for their shows, but Star Trek was an odd animal that had fairly low affiliate coverage and difficulty landing consistent sponsorship, and there was some conflict between the programming and the sales side at the network. They had roughly 33 prime time shows per year to slot, and it was impossible to give each one its ideal time slot and ideal lead-in.
Shows with better ratings than Trek were often canceled, so the fact it stayed on the air for three years was something of a miracle. The Monkees was never a huge ratings hit but its ancillary revenues (let's not go into Monkees business again, please) made it worth keeping on the air for a while. Gilligan always pulled in higher numbers than Trek but its appeal was strongest with children, so it didn't command great ad rates.
Re: blaming NBC for Star Trek's troubles, there's one thing I always found funny. In 1970s fandom, and I'm thinking of David Gerrold or the Lichtenberg-Marshak-Winston book, or both, somewhere in there, were the writers who complained that NBC must have picked "Spock's Brain" to debut the third season as a way to sabotage Star Trek. Like, "That's what they thought of Star Trek!" But NBC didn't write and produce the episode. It just didn't occur to "official fandom" in those days to blame the show itself for its own failings.
The days of painting NBC the villain was also the Roddenberry as God era, so there you go.