Curious as to why you think Libertarian means no Democrat would vote for him. A Libertarian is completely for open borders, welcoming everyone and anyone. That's a big match for Dems. Groups like Antifa, which are heavily Dems, would be a very close match to Libertarian ideals. If he ran, which I really doubt he will, I think he'd pull more from Dem voters than Republican voters.
Also, there is a movement from the Bernie Bros to try to get a 15% threshold for a 3rd party candidate. A lot of Dems are not behind Biden. If they can get to the 15% threshold, then that means next election that party will get gov't funds for the campaign, and be included in debates. I'm not saying it'll happen (it probably won't), but there's certainly more talk of a third party this year than the last go round. If Bernie wanted, he could start his own party and get to the 15% vote total. That'd be a legacy move for him. Note that he has not released his delegates.