The term limits movement of almost two decades ago latched onto a fundamental truth about human nature and politics, to wit, when people stay too long in power, they tend to get rusty, bored and corrupt. They see themselves as politically immortal, when their own feet are just as clay-caked as anyone else's. At this point, what would refresh them better than rest -- a change of scenery and vocation.
Cincinnatus, back in the fifth century B.C., had it about right when twice he accepted an invitation to become dictator during local emergencies, then, when everything was under control, resigned -- went back to the plow he had earlier left. It was a precedent that George Washington followed, consciously perhaps, when he returned to Mount Vernon upon helping the new republic launch itself.
Renunciation is the virtue that slashes like a kitchen knife when seized. Members of Congress, immersed in their privileges and perquisites, aren't the renouncing kind. Aides, lobbyists, reporters, sycophants of one sort or another give Sen. A or Congressman B the most subversive gift possible -- the big head. Yes, sir (it goes), he's the man, she's the woman, gotta stay in there, can't quit now, no, can't quit ever, where's that phone, got to make some fundraising calls.
A term limits law, or constitutional amendment, wouldn't save the country from egoism, stupidity and the lust for eternal power (cf. California). It might at that mitigate the severest consequences of eternal staying on, in the manner of West Virginia's 92-year-old Robert Byrd or, for that matter, Ted Kennedy, Massachusetts' permanent senator until the divine quorum call reached him after 47 years.
As the old saying goes, there oughta be a law. Really.