From the summary Tron:Legacy grossed $400 million from $170 million - that's a success even by Hollywood "hide the profits to avoid royalty payments" maths.
Eh, it's closer than that. Studios see 50-55% of the domestic box office and anywhere between 15-50% of the foreign box office (depending on how much of a cut the distributor in a particular country takes.)
Tron 2 did $172 mil domestically (so figure the studio got $86-$95 mil) and $228 mil internationally (so that take isn't going to be any higher than $114 mil MAX, and probably a lot lower.) So that's $209 million at the very top end, against a production budget of $170 mil. If that $170 mil includes marketing costs (everyone reports that differently) then you're looking at anything from a mild profit to a mild loss overall. If you figure an additional $10-20 million was spent on marketing, we're a lot closer to "losing money no matter what" territory. Yes DVD sales will help them profit in the long run, but a success or failure is generally measured at the box office.
I can see them doing this - there's no indication that Tron 3 would make _more_ money than Tron 2 did at the box office, and it could very well make less. They've weathered the nuclear bombs of John Carter, The Lone Ranger, and now Tomorrowland all in the past couple of years - it makes sense that they're going to stick with franchises where they can print money.