> quickie comments on HN should be taken more as mental stimulation and kickoff points for further discussion
Indeed.
Setting aside variable details like time frames and cost structures which can be debated separately, what I found interesting about your suggestion is it's a mechanism to create an escalating incentive for copyright holders to relinquish copyrights even sooner than the standard copyright period. Currently, no matter what the term length, it costs nothing to sit on a copyright until it expires - so everyone does - even if they never do anything with the copyright. And the copyright exists even if the company goes bankrupt or the copyright holder dies. Thus we end up with zombie copyrights which keep lurking in the dark for works which are almost certainly abandon-ware or orphan-ware simply because our current system defaults to one-and-done granting of "life of the inventor + 70 years" for everything.
Obviously, we should dramatically shorten the standard copyright length but no matter what we shorten it to (10, 15, 20 yrs etc) we should consider requiring some recurring renewal before expiration as a separate idea. Even if it's just paying a small processing fee and sending in simple DIY form, it sets the do-nothing-default to "auto-expire" for things the inventor doesn't care about (and may even have forgotten about). That's a net benefit to society we should evaluate separately from debates about term lengths.
I see your suggestion about automatically escalating the cost of recurring renewal as another separate layer worth considering on its own merits. My guess would be just requiring any recurring renewal would cause around half of all copyrights to auto-expire before reaching their full term - even if the renewal stayed $10. The idea of having recurring renewal costs escalate, regardless of when the escalation kicks in, or how much it escalates, is a mechanism which could achieve even more net positive societal benefits by increasing the incentive to relinquish copyrights sooner.