I think for Brook, even if he regrets it now, he has to still see it as a smart decision. With his injury history he's got to the take the money when it's offered.
As for Thad, he may end up regretting the decision. If he were a free agent this summer he could probably get $15 million a year from somebody, easily.
As for the basketball situation, the Nets are in a dismal spot, but if they're willing to spend some money this summer they may very well be in the mix for one of the last playoffs spots in the East next year. It's probably better for Thad than being on the Sixers, at least.