I always felt like this case was argued wrong. Don't argue it's not "limited", argue that it "doesn't promote the progress of the Sciences and Useful Arts".

There is no possible argument not involving time travel by which we get more works created through retroactive copyright extension of already existing works.

Easily dismissed by pointing to the vast advances in scientific and artistic output over the last century. Unless you could show that the rate of output is slowing (which it isn't AFAIK) you're stuck with trying to argue that it would be even better if copyright law were abridged - a slippery pole argument, if you will. Courts typically eschew such hypothetical benefits in favor of rectifying measurable injuries.

Fair enough. My real point though is that the Constitution says that the justification of copyright is to promote the useful arts and sciences.