To reach the conclusion you did, you must be in possession of some especially enlightened knowledge that permits you to know that extending a copyright an additional 20 years is a violation of the term "limited." There are others on the thread who have been advised by their own inner voices that the proper term is 50 years, or 28, or some other number. So the issue quickly ceases to be what the right number is, but rather how we decide whose inner voices shall prevail.
The Supreme Court has essentially ruled here that whatever their own inner voices tell them is the right number, they do not under the Constitution have the right to impose their own views. The mechanism provided is that the elected represenatives will hash it out during legislative debate, and whatever number comes out of that process is the law, whether anyone thinks that is the right number or not.
You apparently do not think that the legislature arrived at the right number. That's fine, but why should your view prevail? Why not mine, or Michael Eisner's, or Richard Stallman's? You seem to want the court to accept your interpretation as correct, and to enforce it by means of law, and you're hanging your hat on the idea that the word "limited" in the Constitution proscribes a 20-year extension. Had the new copyright saw said "extended in perpetuity," I could see your point. But 20 years is not "forever," and you can't make it mean that just by wanting to.
I think some people here are trying to debate the number instead of the mechanism by which the dispute is settled. They don't like the number that Congress arrived at, so -- like the plaintiffs -- they want the court to decide out of thin air that the legislature got the number wrong. If the court can do that, then the court can decide that a law that says "65 miles per hour" should really say "58.7" or "90" or whatever the Hell they feel like. Why bother with elected legislatures at all if we're going to have rule by philospher kings?
A million years isn't "forever" either. Let's have million-year copyright terms. Sound good?
The Court was stuck, however, by the fact that this loophole doesn't involve an obvious circumvention of the "limited terms" clause at any specific step. At best, the Court might find a "clear pattern" rationale if Congress extends and re-extends copyright terms a few more times. If so, the issue might be resolved sometime in the latter half of the twenty-first century.