I think the key is he's in a local position. Point is no matter what, his "replacement" has to run and win. Point is, he helps good people win wherever they happen to be. For the most part senator isn't an entry level job. Right now I'd say it's fair to say Bernie is to my knowledge the best currently sitting senator, if I'm missing something blatant he's at minimum top 5.
Senate isn't exactly a position where you can pick an heir apparent, and for the most part people can only win with experience winning a mayorship, governor or house of representatives seat first. Fact is there's 99 senate seats with people not as good as him to replace, 465 house seats, 50 governors, tons of mayorships etc... that he can throw his weight at besides his own. Yes it would be great if there was a 40ish year old bright and scrappy contender for senate in vermont that bernies endorsement would make a shoe in... but to my knowledge there isn't.