Not the dead guy, the Act.
First, let's take a look at the relevant part of the Constitution (emphasis mine):
The Congress shall have Power [...] To promote the Progress of Science and useful Arts, by securing for limited Times to Authors and Inventors the exclusive Right to their respective Writings and Discoveries;
Now, notice right away that the Act is retroactive to works already published. That means Mickey Mouse gets twenty more years of copyright. It is not difficult to conjecture that Congress might, in twenty more years, extend copyright another twenty years. Then they might do it again. Obviously, if they did, forever, copyright would not be secured for a 'limited time.'
'But they haven't!' says the Supreme Court. Of course they haven't done it forever, dummies. And they never will. That's the way forever works. Forever is the infinity of time. So let's look at infinity.
Infinity, colloquially speaking, is where you can always add one more. (Or twenty more.) So, can Congress always add one more? More precisely, will the Court always let them add one more?
Suppose the Court allows Congress to make an unlimited number of limited-time extensions to the copyright term. Clearly, this is not a 'limited time.' We need not actually reach infinity to be unlimited; we merely must have no limit.
Since the Court cannot allow an unlimited number of extensions, it must therefore at some point limit the number of extensions and, consequently, the number of years. One day the Court has to draw the line. '99 years is enough,' perhaps. Hold it! Go back to Constitution. Only Congress has the power to decide the term of copyright. The Supreme Court does not.
We're left with an inductive proof by contradiction: If any retroactive extension to copyright were constitutional, then an unlimited number of them also would be constitutional, which is, of course, unconstitutional. Therefore, no retroactive extension to copyright is constitutional.