Assuming if Jeb doesn't want to be President, it would be the perfect way to cap off his career (a la Bob Graham).
Even if he does, he certainly isn't going to get to be President by just doing nothing for the next 6-10 years.
The 'Senators Can't Get Elected' crap is a myth. The last 3 that have tried have been total yutzes, didn't have anything to do with being a Senator IMHO.