From the tone of your post, you seem to think this is a problem. If so, why?
If we really care about getting the content released to the commons as early as possible, just apply a single shortened date to everyone.
I don't have a problem with 'buy an extension to copyright', though - that money can fund public goods that benefit everyone.
It is as large an obstacle to the hoarding and disappearing of content corporations do today as any alternative, so I'd go for the easiest, whatever that is.
Why make it complicated? Just let works fall into the public domain after a couple decades.
If they price they quoted was fair, they should be more than happy to sell it for twice that. If it was too low, well shouldn't have lied. If it is too high, more money to the public.
Meanwhile, derivative like works could still be created as long as they were clear to not confuse consumers.
Look at patent registry
https://www.dpma.de/docs/english/formulare/allg_eng/a9510_1....
Even if Mario entered into public domain, Nintendo Mario games would still be the superior choice. It isn't like people would be allowed to claim their game was an official Nintendo Mario game. Profitable IPs would also be able to grow their IP as a means of protection. For example, there would be a significant time lag between when Mario would enter the public domain and Yoshi would, and as such more recent Nintendo games couldn't be copied because they include Yoshi (replace Yoshi with dozens of even newer characters).
Edit: One way to think of it is that others could only use the cannon as it existed 30 years ago. As long as the IP isn't dropped and fans like where it is going, that provides a significant barrier of protection from others branching off and making their own cannon.
EA being able to freely make "Battlefield Mushroom Kingdom III: Micropayments Edition" would not be a good thing.
FTR, The source code for Super Mario Bros. X, a well-known Mario fangame, is now open under the MIT license.[0,1]
[0] https://www.supermariobrosx.org/forums/viewtopic.php?t=25160
[1] https://old.reddit.com/r/opensourcegames/comments/f8tira/the...
After that, renewals should occur every 5 years and get expensive.
The goal is to encourage those who can create to do so--not create a rental property in perpetuity for corporations or estates.
I don't really get this. If someone spends 6 months, or even a year or two, writing a book, why should they be entitled to profits from the sale of that book for the rest of their lives? That seems a little excessive to me.
Because, in spite of what the Internet thinks, good creators are rare and we should wish to encourage them.
In addition, good creators often aren't recognized immediately--there is time lag. Corporations shouldn't get to profit from your labor of 20 years simply because it took that long for you to become popular.
Mickey Mouse is much more relevant, would lead to much more creative work, and would hurt Disney much less if it were public than making some song by some band which at best has a small dedicated following public.