So corporations can renew copyrights but not most individuals in practice. That seems problematic. Because, I don't know, the cards are already so stacked against individual content creators we don't want to make things worse?

If we really care about getting the content released to the commons as early as possible, just apply a single shortened date to everyone.

On the other hand, the more valuable content can in commercial use for longer.

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.

I think the argument is that a corporate entity would be willing to lose a large amount of money by extending copyrights on their back catalog, if only to keep those works from competing with their current offerings that make them loads of money.

A strong enough exponent, and the costs of holding the old works back would very quickly eclipse the protected profits.

A single shortened date is a good start.

I don't have a problem with 'buy an extension to copyright', though - that money can fund public goods that benefit everyone.