My feeling is that copyrights should be infinitely renewable, with say a 20 year term, but the renewal fee should double with each term so that Disney can have their infinite copyright on Snow White but at an ever-increasing cost so that they will need to make a decision about whether it makes sense to keep it.
My utopian vision: First registration is free and automatic. Copyright holders get an automated notification of expiring copyright and renewal is, say $1000 for the first term (adjusting for inflation) and doubling thereafter (also adjusting for inflation, so you don’t get a $2000 renewal but more like $4400 with 4% inflation). For corporate-held and posthumous extensions, the term would be 10 years.
The thing here is the exponentially increasing cost of renewing the copyright, with the inflation-adjustment I proposed and the 10-year term for a corporate or posthumous copyright, in 90 years, that $1000 renewal fee goes to $256000 in inflation-adjusted dollars. 110 years it’s a million dollars, 210 years it’s a billion dollars. If a work is worth spending that kind of money on renewing the copyright, why not let them keep it?