I think about my fundamentalist feelings about copyright (between complete abolition and say 20 years like you suggest) and the practical reality of how that would fundamentally change content industries today. For example, I'm thinking of anime franchises, also Disney, etc, who essentially create whole industries out of remakes and repackaging their old IP. Essentially, millions of workers hired to maintain the less creative recreation/maintenance of these IPs would be reshuffled and or lost. Btw, this isn't just creative workers like writers, animators, and the like, but things like merchandise creation, manufacturing, live events, and so on, they'd all be much less of an incentive to maintain a
franchise over the mere creation of works that might be separate from each other.
Apart from the business/worker side, the ability for an IP holder to release continuing sequels to works (like manga, movie installments, etc) and the legal force against fan works essentially gives more power essentially to the original creator to shape what is "canon" in their work. Perhaps fans could still decide the original creator's later installments are canon, but the extra legal force sort of makes this defacto, while a world of limited copyright the power of the original creator is less so. Thus, the way fans consume media and experience it will change completely as well.
It would just be a completely different world, one where a lot of these ills are addressed (like the ones mentioned by GP), and things will be very different. It's hard to say from my vantage point whether the differences would also be "ills" or not (and that might be my bias), but it would certainly be a different world with different rules and different expectations from fans and audiences.