We actually took legal advice on this at one point, and the bottom line was that we could well have a claim against the reseller (as they were the other party in the actual sale contract) if something unfortunate happened, but that's only worth anything if the reseller themselves is still around. Our major concern here is that software we've paid for that might otherwise be useful indefinitely could be rendered useless because some external system ceased to be available.

This being the case, relying on the ability to sue anyone and get meaningful compensation after the fact seems unwise. Clearly some sort of legal obligation for those selling the software to make arrangements in advance to unlock everything if they go under would be a much more useful guarantee for customers. It would suffice to have a version of the software sold that is identical except for removing all copy protection logic, and require that this be provided to a third party escrow service to be released automatically in the even that the software developer or their agent/successor is no longer providing the necessary services to support that copy protection system.