Twenty years sounds fair to me.
Twenty years from creation (copyright is currently defined as starting at creation) is way too short. I'm about to publish a trilogy of novels, and I started on the first one in 2001. By your standards, six years of profiting from my works should be enough. That's laughable.
You wrote the first one in 2001 and sat on it for 13 years, then complain that 20 years would be too short?
Twenty years from first publication might be reasonable
20 years from publication is about twice as long as is reasonable. Most novels make the vast majority of their sales in their first year, after that it just peters out to nothing over a number of years. It's a rare novel indeed that still makes sales after ten years, let alone twenty.
On the one hand, you have individuals creating works, and on the other hand, you have big corporations creating works.
That one is easy - disallow corporations from owning copyrights. There's no sane reason why copyrights should be allowed to be transferred from the creator anyway - and a corporation is not an entity that can create things anyway.
Either way, a flat twenty years is absurd.
No, what's absurd is the current situation of life+95, with renewals allowed every time that term is in danger of coming to an end.
Copyright is supposed to be a restriction of our right to copy the works of others so that the other can profit from it for a short while - thereby giving the other an incentive to create. But giving up our right to copy forever was never the intention of the deal.
It's high time to renege on a deal that's been perverted by one side into something that no longer even resembles the original intent.