term limits for the supreme court

Peggy Noonan is suggesting that we impose term limits on our Supreme Court justices:

Supreme Court justices are more powerful than ever while who and what they are is more mysterious than ever. We have a two part problem. The first is that no one knows what they think until they're there. The other is that they're there forever.

I find myself lately not passionately supporting or opposing any particular nominee. But I'd give a great deal to see Supreme Court justices term-limited. They should be picked not for life but for a specific term of specific length, and then be released back into the community. This would involve amending the Constitution. Why not? We'd amend it to ban flag-burning, even though a fool burning a flag can't possibly harm our country. But a Kelo decision and a court unrebuked for it can really tear the fabric of a nation.

I understand the desire to reign in a Court that sometimes makes unpopular decisions. I think it's a misguided desire, but it's nonetheless understandable. What I don't understand is the assertion that limiting justices to a single term of fixed length would somehow motivate the justices to clean up their act. What new incentives would term limits provide to justices? Under the current system, a justice will lose her position when she's dead, regardless of her performance on the bench. Under Noonan's proposed system, a justice will lose her proposed position when her term expires, regardless of her performance on the bench. This changes the justice's incentives not one whit. Am I missing something?