It doesn’t matter if they created the work 20 years ago or not, it’s theirs. Can people go into your house 20 years after you build it and just say it’s their because they like the way it looks? Intellectual property is still property.
If you charged people $5 for a lifetime pass to hang out in and inspect your house, should they not be able to try to build their own version of your house 20 years later?
When you enter something into the culture, part of it leaves your control, and thats a good thing.