It seems more often than not that the incentive is just to sit on the rights to IP, even if you don't plan to do anything with it ever, just because there's no incentive to let things go, and there's some microscopic but non-zero benefit to holding onto IP just in case.

I wonder if, instead of having the near-endless wait until public domain, it would be good to have a system of intellectual property that made copyright increasingly expensive to hold year after year, as some function of the total assets of the IP holder. Make it some exponential function, so that after a certain number of years, holding onto the rights to some IP is just too expensive to maintain unless the holders feel they can actually do something good and profitable with the IP (rather than a system that would need to show regular "use", which would incentivize publishing crap to satisfy a regulation).