Personally, and perhaps I just can't think of any at the moment, but I don't believe I have any examples for this case.
A TV series stretching itself too thin and straying from the original idea, I feel, doesn't have to do with it being a series. It has to do with the fact that they decided their series needed to a full-length one. If they stopped the show after how many episodes they needed just to tell the story they came to tell, then that wouldn't be a problem.
Movies do allow this to be a restraint in of itself, but it being a tv series shouldn't stop the story writers from keeping their tale as short as it was originally planned to be.