as I've pointed out before, if he ran as an Indy spoiler he would face retribution from the Dem Senate caucus re: committee assignments. they could turn him into a backbench ghost, and it's not like he can caucus with the GOP, so he'd have zero leverage.
so even if he genuinely doesn't give a sh**t in a Clinton v Bush race he will not run as an Ind.
he's 73. how many more senate terms does he have left anyhow? if he ever wanted to go all in, now would be the time.
as Wulfric below me pointed out, 73 is hardly all that old for a Senator. he's a lock to be re-elected in 2018, and thus serve until early 2025.
he's also not one of these Senators who can walk out into some six-figure private sector job, and now that Feingold's out he's probably near the bottom in personal net worth among all of the Senators. those six more paychecks of ~$175k have meaning to him.
Well, he'll be 77 by 2018. But still if he wants to run for re-election, he won't run as an Independent.