Justice, we are not making an empirical claim at all. Nothing in our Copyright Clause claim hangs upon the empirical assertion about impeding progress. Our only argument is, this is a structural limit necessary to assure that what would be an effectively perpetual term not be permitted under the copyright laws.
this is a (related to what holds something together and makes it strong) limit necessary to promise to that what would be an effectively constant (forever) term not be permitted under the copyright laws.