Okay, this will be my last piece said about this because I don't want to go round and round.
TMP with it's various issues of writing quality, had itself a sequel, if the studio would spend the equivalent (or more) of the same budget, they could do another movie with the large quantity of special effects and it could quite possibly be successful, maybe even get another sequel after that. Given the issues with TMP, the franchise was going to lose Nimoy in the sequel, which also probably would have led to other departures in subsequent films. Taking piece after piece away from Trek until it was very little about the characters. (Yes, a slippery slope argument, but given the course set at TMP, it's not an unreasonable one)
The studio didn't want to spend the huge budget for Trek II that they spent for TMP, so it required a change of course. The more character driven course, attributed somewhat substantially to Bennett, kept the original cast together and produced some of the finest hours of Trek ever produced.
I agree with you that the franchise wouldn't have just died without him, but he helped ensure it remained a quality product. I think there is no doubt that without certain individuals, including him, Trek would be far different than it is.
As a final note, can we just be nice to the recently deceased? I see nothing wrong with saying that the guy was a savior of Trek. When someone passes away, there are tall tales told, some of them true, some of them exaggerated, probably some really exaggerated, but this shouldn't be offensive unless we were to credit him with creating the franchise in the first place or inventing the modern computer.
Mr. Bennett, may you rest in peace, Sir. Thank you for your hard work on a franchise we love so much.