Barring a full-blown revolution won't that always be the case?
Isn't that already the case? Look at Ginsberg's last wish. Also the very same argument could be used for the other branches of government. So no term limit for the president or congressmen also, I mean their experience can't exactly be replicated. Hm, I think not.
Besides I'm not saying that retired justices couldn't advise on matters they have such great expertise on, it could happen if the current justices thought it prudent, but to place so much on a geriatric club seems both inefficient and cruel to me.