https://unclemikesmusings.blogspot.com/2016/09/top-5-reasons-you-cant-blame-nbc-for.html
5. The Sale of Desilu Productions.
4. 2001: A Space Odyssey.
3. Fred Freiberger.
2. Political Correctness -- 1960s Style.
1. The Apollo 1 Fire.
Those reasons are mostly nonsense, although Freiberger's ineptness undoubtedly played a role in the loss of quality, which led to lower ratings.
As for McCoy's swearing, he most definitely did say not only "Damn it, Jim!" but "Damn it, Spock!".
NBC was and is a business about making a profit. My understanding is that the profit made on Star Trek was minimal at best. If you are truly counting the beans then why not spend on something that promises big returns even if a sizable portion of America would gag on it?
Gene Roddenberry. In hindsight we can see that Gene got out ahead of his critics by presenting material designed to put Roddenberry in a good light. Now we know that Gene had a fair amount of flaws including trying to bilk composer Alexander Courage out of what he was due for his work. Often promised one thing to writers (mostly during Season 1) and did another. Kept allegations of sexually inappropriate behavior out of view and his promotion of Nichelle Nichols and Majel Barrett was due to them being mostly his consorts at one time. Gene did not have the best relations with NBC in terms of meeting schedules and so forth. The Corbomite Maneuver was intended to be the regular season pilot but was not delivered until episode 10 was to be aired. Gene was stingy with credit and Gene Coon's contribution was not noted for quite some time after the show ran. NBC got tired of a problematic producer who's show made very little money for the network who was anything but a flagship operation then.
I love the show but it did have its share of warts both on and off the screen.