I'd be open to there being an additional cost based on total revenue earned, but the scale should be the fixed costs, with significant additional revenue just moving that number up.
I think the goal for the financial cost of the extension to be roughly equal to the "harm" caused to the rest of society for locking it up. So, I think making the scale in some way proportional to revenue could be a rough proxy for "how popular is this thing", which is then a rough proxy for "how much does it harm society to lock it up for 5 more years".
That said, I'm more interested in being equitable to _society_, rather than being equitable to _creator_. Copyright should be just good enough of a deal that it encourages creative works. It doesn't need to do more than that.
I think the goal for the financial cost of the extension to be roughly equal to the "harm" caused to the rest of society for locking it up. So, I think making the scale in some way proportional to revenue could be a rough proxy for "how popular is this thing", which is then a rough proxy for "how much does it harm society to lock it up for 5 more years".
That said, I'm more interested in being equitable to _society_, rather than being equitable to _creator_. Copyright should be just good enough of a deal that it encourages creative works. It doesn't need to do more than that.