I think there's a fairly simple solution.
Copyright may be held up to the current limits (so no one can complain that "their rights are being taken away"). But you need to pay each year, with the cost increasing exponentially every X years.
First year is free. For most people, this would include the vast majority of any income they will get from the work. Next say 5 years is a low cost. If you're making any kind of ongoing revenue then this payment should be easy. And every 5 years the cost doubles for the next 5 years.
So if Disney wants to keep copyright up to the current limit they can, but at some point they have to make the decision that it's no longer cost-effective. Meanwhile, most other works would have entered into the public domain much closer to the original 20 years (and in many cases after only a couple of years).
Year 0 - free.
Year 1-5 - $1000/year
Year 6-10 - $2000/year
Year 11-15 - $4000/year
Year 16-20 - $8000/year
Year 21-25 - $16000/year
Year 26-30 - $32000/year
Year 31-35 - $64000/year
Year 36-40 - $128000/year
Year 41-45 - $256000/year
Year 46-50 - $512000/year
Year 51-55 - $1024000/year
Year 56-60 - $2048000/year
Year 61-65 - $4096000/year
Year 66-70 - $8192000/year ...