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.
4 million people are neither happy or sad  with the amendment
( z1 or :op ( z2 happy :polarity - :ARG1 ( z3 person :quant 4000000 ) ) :op ( z4 sad :polarity - :ARG1 z3 ) :ARG1-of ( z5 cause :ARG0 ( z6 amend ) ) )