So, yeah, give me a break. I long for the moment that there's a much more honest treatment of Roddenberry and what he REALLY did for (and against) Trek over the years.
Isn't the Justman / Solow book good enough? They are pretty up front about Roddenberry's faults, but also credit him where due. The way you're painting it, Roddenberry had the dumb luck to come up with a TV show idea that other people made great.
The thing is, people react so strongly in their efforts to knock the man down a few pegs, that they whittle his contributions to almost nothing. That's no better.
Star Trek was not cancelled because of bad ratings, shows were saved and carried over with much worse. Star Trek was cancelled because the ratings weren't high enough to put up with the crap that the show's staff kept piling out. If Roddenberry had tried to stay on for the third season, there would not have been a third season.
I'd love to see your source for this, since every other word on the subject is that Trek was given a third year and Roddenberry left over the time slot change. Nobody forced him out. He did it all on his own, to his own detriment.
Star Trek was cancelled because it was not making money for the network. Roddenberry was already long gone and the scripts were lacking, even the fans were watching other stuff. Say what you want about his treatment of women and his need to grab credit, but it seems odd that the show's script quality would take such a nosedive as soon as he left, since you seem to be saying he wasn't responsible for any of Trek's success.
Star Trek's success was a major team effort, but Roddenberry was a huge part of that team.