I have asked a reseller about how this operates if the original developer of the $X,000/seat software my business relies on goes under, and they basically laughed my question off, saying that would never happen. I have also, in an entirely different context, had some knowledge of legal actions involving that same developer, and I don't share the reseller's faith in their guaranteed longevity.
There have been occasional court cases on these matters, and at least in Europe the courts mostly seem to have taken the view that if it looks like a purchase (for example, if there was a reasonable expectation that once money changed hands the new holder of the software would be able to use it indefinitely) then it must be respected as such.
Still, I wonder at what point primary legislation is going to become necessary. Copyright reform has been vigorously opposed by Big Media and mostly successfully so far, but when you start hitting large numbers of customers in the wallet and they can't just pirate their way out of it and get away with it any more, sooner or later I suspect something has to give.