I wouldn't escalate on disuse, just straight-up time. Exponential I chose to make it cheap to hold on to for a couple of decades, but by the time a company is claiming something for the 80th or 90th year, they better be doing something pretty darned awesome and profitable with it to justify keeping it out of the public domain.

And if they are... I really don't mind. I'm a bit less incensed at the fundamental idea of "culture being locked up"; if something is being maintained as Star Wars is, for instance, let Disney have it for as long as they can homestead it, basically. I'm personally far more aggrieved about the pile of stuff that nobody owns and nobody particularly wants, but is locked up because Disney wants to own Steamboat Willy. My pragmatic solution is to basically let them, but to do so with much, much less collateral damage.