Given the number of senators who have expired in office lately (Inouye, Kennedy, Byrd, Launtenberg) or might have (Specter had he won another term, perhaps Lugar if he hadn’t been sacked in the primary, and Orrin Hatch and John McCain seem to be going for this, too), I think Castle might have been another lifer. The fact that he would run for Senate at 71 and after suffering two minor strokes back in 06 suggested he didn’t care he was way past his prime (not unlike Tommy Thompson in WI), he just saw what looked like an easy route to the Senate and took it. Odds are he’d have served until he came out with a toe tag.
With respect to Castle, that wasn’t the impression I got. I said I only believed he would serve part of the current term (with the Dems “allowing” him to serve it) and retire to have Biden, Jr. to succeed. I don’t believe Coons was considered a serious candidate until Castle went down (and now Biden, Jr. has nowhere to go unless he wants to wait for the now dead-end job of Governor or challenges one of the other Dems, which I’d be surprised if he does).