Copyright was setup by the people to give incentive to create new works for the people. If you can rest on the work you did decades ago, there leaves no reason to continue with the arts†, which goes against the whole spirit of copyright in the first place.
† Assuming profit is the driving motivator, as copyright does.
I think you're putting too much emphasis on trying to maximize output from each individual creator. When, in every case I've ever heard or read of, creators already go as fast as they can (provided they can afford to live off their creations). And, further, despite those efforts very few find subsequent success.
So worrying that creators might sit by the proverbial poolside instead of creating new works seems silly to me and completely at odds with reality, even under our current copyright regime where copyright is effectively perpetual.
What I take into consideration when thinking about copyright terms, is trying to balance a reasonable time from which a creator can make a living off their work (given what we know about how creation happens), against a reasonable time before which others can try to make their living from that work.
As that is also what the public domain is about: freeing up old works for others to extend, adapt and interpret in their own attempt to make a living.
And along those lines, it's worth recognizing that there's a very real risk to the rate of 'new' creation that would come from shorter copyright terms. Should Mickey Mouse fall out of copyright, wouldn't the corporate incentive be to put Mickey Mouse into every children's work? (Particularly given the state of our risk-averse creative industries.)
The 'new' works we got might well start looking even more familiar than they do today.
> So worrying that creators might sit by the proverbial poolside instead of creating new works seems silly to me and completely at odds with reality...
It seems equally silly to worry about squeezing every ounce of profitability out of every work of art that we can think of, and for every edge case.
Artists will keep creating whether copyright is infinite or only 14 years. Maybe 22.7 years is optimal, but this is a boring discussion to have
> balance a reasonable time from which a creator can make a living off their work (given what we know about how creation happens), against a reasonable time before which others can try to make their living from that work.
Perhaps shift the perspective a little. What is reasonable time period for which others should be prevented from producing new work derived from the original? The problem I have is that copyright is at conflict with what we all know to be reality: most art is a product of our shared culture and only partly original. Copyright actually blocks the natural evolution of ideas and prevents society from benefiting from significant derived works.
† Assuming profit is the driving motivator, as copyright does.