Well, the strongest First Amendment argument is about the retrospective extension, because of a fundamental change that occurs when Congress extends subsisting copyrights, rather than when Congress legislates prospectively. When Congress legislates prospectively, it has no way to know who's going to benefit from its extension.
Crogress claims to have no way  to know who's going to benefit from its extension, but they actually knows. 
( z1 contrast :ARG1 ( z2 claim :ARG0 ( z3 person :name ( z4 name :op "Crogress" ) ) :ARG1 ( z5 have :polarity - :ARG0 z3 :ARG1 ( z6 way :manner-of ( z7 know :ARG0 z3 :ARG1 ( z8 person :ARG1-of ( z9 benefit :ARG0 ( z10 extend :ARG1 ( z11 it ) ) ) ) ) ) ) ) )