premise: Whereas if you don't accept the Copyright Clause claim, then, in order to make the First Amendment analysis we've simply got to say, well, gee, is the promotion of useful art and so on more important than the public domain, and can we say that that allows a distinction between 50 years and 70 years?
hypothesis: The First Amendment is the only one that matters.
reason: The First amendment is the only amendment stated in the context, but it's not stated anywhere that it's the only one that matters.