The present system is fine. I do not support changing it.
I would be interested in a study of senators and representatives seeking re-election after being in office for several terms. Consider the likes of Ted Kennedy and Ted Stevens. Assuming they choose to run for re-election, what chance is there that they could be defeated.
Well, clearly the fact that the person has demonstrated their popularity by virtue of being reelected frequently in the past would make them more likely to be reelected in the future.