The question is: What's the shortest term that would enable most creators to capture most of the potential income from their works? That's what it should be, and not much more.
Yes, that's the original idea. The old system was good because the author got one more 28-year inning if he felt like going to the trouble of filling out a form. Usually, this didn't happen -- in the author's mind, the work was already abandonware by then. If we'd kept the old system, non-renewed works written in 1981 would be entering the public domain today. To attempt to find the right term, you'd want to look not only at the income curve but the abandonware curve -- how long it took the artist to become convinced that the work had no further financial potential. You can't alter copyright law successfully if you freak out all the artists.
Nondeterminism means never having to say you are wrong.