You apparently haven't been paying attention for the past few decades where except for the occasional wave of discontent incumbents remain in office for far too long.
Why is some scumbag like Foley or Sherwood or Ney hanging out so long? Why do I have to listen to Kennedy or Schumer or ... ad nauseum ... any more?
The congress should be like the military: either you move up or you move out. If a congressman can't get himself elected to the senate after a few terms in the House he should go back to being a scumbag lawyer. If a senator can't get himself elected to be governor or president he should go back to being a scumbag used car salesman.
Couple of things.
First, the scumbags like the ones you mention stay in office because the people of their districts or state like to be represented by scumbags. People like Kennedy or Schumer simply reflect the character of the electorate in their respective states. One need only reflect on the congressional delegation from Mass to know thats true. They are, almost to a person, as much or more of a scumbag as Kennedy.
Secondly, many would not consider the Senate to be a promotion from the House nor a Governorship a promotion from the Senate.
Third, term limits often result in the throwing the baby out with the bath water. Good legislators get ousted with the bad. I know a couple of excellent Oregon legislators who had to leave the Oregon House because of term limits. Had they stayed they could have provided important leadership and influence on a host of conservative issues.
Instead of a blanket term limits, a better approach is to find a way to selectively cull the herd by voting the scum bags out of office and that is why I suggest that we already have term limits.