I think the show's legacy would be tarnished a bit. People just can't let go. The series is over, folks. Done. Get over it. If the decision to bring the show back for a one time movie is exclusively money motivated, then I doubt the quality of the movie would be that good to begin with.
The Sopranos was an ongoing series of episodes over a lengthy timeline. What could they possibly cram into a two hour movie that could satisy the diehards?
Besides, most of the good characters were killed off in the show's last season.
Christopher, Bobby Bacula, Silvio are dead. Uncle Junior is a vegetable now. The characters left over are boring, with the exception of Paulie Walnuts, Carmella and Tony. Who cares about annoying Meadow, Janice, and that little pussy, AJ?