The archival angle works for Unity because the actual game code is run in a .NET CLR (Mono in most variants of the engine).
Maybe the best way forward is source code escrow the way Trolltech arranged with Qt. They had a deal where the if they ever stopped working on an open source Qt, the last release would become automatically BSD- or MIT-licensed and thus available to all. They had the details worked out nicely. I really believe that Unity should be pressured into such a deal. It is not important now, but it will be an entirely different thing 5 to 10 years after they fold (if ever).
