That's the compromise. Disney gets "The Mouse" for the entirety of its existence as a legal entity, and six more works. And that's it. After that, they'll need to start being thrify with their copyright or release things into the public domain to add new works.
I think you are missing my point. Essentially Disney will start buying your lifetime copyrights. Given that there are 300MM Americans, I'd imagine if every American got 2 lifetime copyrights, the market for transferring those copyrights wouldn't be terribly expensive. Disney could probably buy 100 lifetime copyrights for the cost of the legal work they do today.
It would be tied to lifetime of the particular Americans. I imagine assassination black market would flourish. :)
Or they could just create shell companies to own the copyrights, allowing them to have an indefinite number of copyrights for forever!