I hate when people jump ship - or especially party - right after the election or serve half of their first term then run for something else but a 6 year term is a long while to assume no life changes and it is a demanding job that can burn someone out and he's been in since the late 90's, house then senate. I prefer people serve their whole term too, but outside that first term I don't thinks it's too big a deal. Lots of our senators die in office and I think you'd have to make the case that a 90 year old shouldn't be seeking re-election if we're categorically ruling out mid term resignations as ethical after the first term. Not ideal but no big deal, I think.