It was not a mistake to make the move to the big screen for TNG. I would even argue that it wasn't a question of whether or not it was the right time. What bugs me about all the TNG films (even First Contact, which I love), is that the characters aren't who they were on the show.
Picard was never a gung-ho action hero, and the dynamic between him and Dr. Crusher is eliminated completely. Hell, we barely see much of Dr. Crusher at all through four movies. Data should NEVER have been given his emotion chip. He was much more fun as the emotionless android constantly trying to understand what it is to be human. And whose idea was it to ignore the team of Data and Geordi, one of the best friendships of the TV series? Generations is the only one of the four movies that really acknowledges it! Meanwhile, TNG movie era Troi is no longer the Counselor Troi from the TV show. Now she's just.... Marina Sirtis.
About the only main character the movies get right is Worf.... and he'd moved on to be a crew member aboard Deep Space Nine by the time of First Contact, the only one of the post-Generations sequels to give a rational explanation for why he'd still be tagging along. Well, Riker pretty much seems like Riker, so I guess he'll do also. But only getting two characters right? That's bad.
Even when the original six TOS films were at their least impressive, I thought the main cast always felt true to who their charcters were.
Agreed. Almost none of the TNG movies play to the strengths of the tv series at all; whereas the TOS movies managed to feel like you were at least watching the same guys from the tv show.
(I always found it ironic that TOS on television was very much centered on the 'power trio' at the expense of the other characters, but the movies gave the rest of the ensemble a bigger share of the pie... by contrast, TNG always had a very strong ensemble feel on tv, but the movies narrowed the focus considerably to just two or three characters.