The old UK system was the longer of 42 years after publication, or 7 years after the death of the author.
The actual numbers might not be perfect, but this kind of system (possibly with different numbers) would eradicate the downside you mention.
It might be appropriate for different types of work to enjoy different copyright terms. You are probably right in saying that 20 years is long enough for a book, but music might last longer due to covers, remixes etc.
It might be also appropriate to have different events occurring after different terms, rather than simply shifting from owned to public domain overnight - e.g.
Up to 30 years: A copyright holder is pretty much god, save for any parody/fair use etc. clauses.
After 30 years: A copyright holder is bound to accede to "reasonable requests", and cannot place unreasonable conditions or fees on use.
After 50 years: Must be properly attributed, but otherwise it is uncontrolled.
After 100 years: Completely public domain, and you can do what you want;
Covers and remixes are no different than derivative works of books, plays or film. Copyright should be universally applied to all creative works.
That said I do like the idea of a potentially staggered coverage. Perhaps 0-X: monopoly. X-Y: compulsory licensing. Y+: attribution
Particularly because music already enjoys compulsory licensing and it seems to work fine. And attribution seems like the right thing to do, regardless of the age of the work.
I don't mean to claim that covers and remixes are any different, merely that the practice is quite prevalent, meaning that tunes penned by various artists 20-50 years ago are still being heard in the charts today, spurring a resurgence in interest in the original.
The same can only be said of a small handful of authors, whose older books lend their names, and occasionally their stories to new films.
I also agree that attribution is right regardless of age; but over time, without a central rights management authority, it could be difficult to assign proper attribution, leading to the potential for copyright trolling akin to the kind of patent trolling we see today. Perhaps instead of expiry, there should be a time immemorial for copyright.
The actual numbers might not be perfect, but this kind of system (possibly with different numbers) would eradicate the downside you mention.
It might be appropriate for different types of work to enjoy different copyright terms. You are probably right in saying that 20 years is long enough for a book, but music might last longer due to covers, remixes etc.
It might be also appropriate to have different events occurring after different terms, rather than simply shifting from owned to public domain overnight - e.g.
Up to 30 years: A copyright holder is pretty much god, save for any parody/fair use etc. clauses.
After 30 years: A copyright holder is bound to accede to "reasonable requests", and cannot place unreasonable conditions or fees on use.
After 50 years: Must be properly attributed, but otherwise it is uncontrolled.
After 100 years: Completely public domain, and you can do what you want;