Cocaine Mitch is a pragmatist, above all. Barring mitigating circumstances, like Trump coming up with overwhelming, unimpeachable evidence of voting fraud, Cocaine Mitch realises that it's probably best to acknowledge Biden as the President-Elect than to challenge the result and, win or lose, put a target on his head.
Remember, we laugh at them for spending whatever it was, a billion or near a billion dollars, trying to get Cocaine Mitch's seat. But, given enough time and money, they could get in enough of their Democrat bad actors to rig it to get them that seat, same as they've done for Georgia. Not only that, but Trump will probably be subject to frivolous, malicious prosecution when he leaves office and, if Cocaine Mitch backs him without good cause, he could easily find himself in the firing line as well
It's a formality. Hillary never really conceded