Agree, Insurrection was no more than a weak television episode and not worthy of a movie.
Insurrection was the tailspin, and Nemesis was the franchise smashing into the ground.
I think it started earlier than Insurrection and Nemesis.
When I first watched Generations on VHS, I thought it was the beginning of the end of the TNG franchise. Generations seemed like a mediocre extended two-part episode. I didn't have any high expectations for further TNG movies.
When First Contact came along, I was pleasantly surprised as to how excellent it was.
By the time Insurrection came along, I was disappointed in how it went back to resembling a mediocre extended two-part episode. When Nemesis came along, I really didn't care anymore.