The result is that you can stay as long as people keep voting you back in, but you lose the incumbency advantage and end up with a higher turnover rate without ending up with a 100% turnover rate. And you make them learn how other parts of the government work. It wouldn't hurt a bit to see long-term members of Congress do a two-year stint in an administrative agency once in a while.
I think I'd suggest a more generous Senate term limit. Three terms (18 years) would allow for someone to see out a complete Presidential super-cycle, for example.
The word Senate is etymologically related to "senior", it's a place where you _want_ people to be able to develop a lot of institutional experience.