Um...Batman Forever made over $20 million more than Batman Returns at the box office.
Merchandising is important for blockbusters, but home video is where most of the money comes from. After that box office is incredibly important. I hope if they have a good Tron 3 in store it gets made, but this is the same argument that was made for Superman Returns, Ang Lee's Hulk, Van Helsing and more. Granted, those were all bad movies and I think T:L is excellent. But the fans of those movies defended their success on the scope of merchandising.
Disney is the king of merchandising and if they decide that it is not worth the $170 million investment ($320 million when you factor in marketing), they may just leave it in animated form and try to find the next big thing instead of making a sequel to a movie that isn't going to cross $200 million domestic after costing nearly a third of a trillion dollars.
P.S. Can I just add that post that is upsetting some was made mostly because I wanted to start a discussion about the movie's ending. Not a box office argument.