Honestly, I'm not that broken up about it. The original TRON was an ambitious and admirable failure, an attempt at doing something groundbreaking that the technology wasn't quite up to; but it didn't really have that great a story, and its appeal is largely nostalgic. TRON Legacy was simultaneously an attempt to cash in on that nostalgia and a step away from many elements of its aesthetic and story; it was a far more conventional action/CGI movie, very stylish in its own way but still relatively ordinary in comparison to what it tried to mimic. And like the original, it didn't really have that much going for it beyond the nostalgia of seeing Bridges and Boxleitner reprise their roles. Uprising was somewhat more interesting and effective at building out the world of the programs, and was also fairly striking stylistically, although it was an acquired taste (I never much cared for Peter Chung's character design style, and the female physiques in the show were rather ridiculous). But overall, TRON isn't one of the great SF franchises. It seemed to me that it had been largely forgotten before Legacy came along.
Sure, it's possible a third movie could've been moderately worthwhile, maybe improved on Legacy in some way or at least resolved Tron's story arc. But I don't feel its loss all that keenly.
Although, as I said, films in development start and stop all the time. Sometimes a project will get cancelled multiple times and then eventually get made. So there's no guarantee that this film is dead forever.
Sure, it's possible a third movie could've been moderately worthwhile, maybe improved on Legacy in some way or at least resolved Tron's story arc. But I don't feel its loss all that keenly.
Although, as I said, films in development start and stop all the time. Sometimes a project will get cancelled multiple times and then eventually get made. So there's no guarantee that this film is dead forever.