If Sanders doesn't run himself, then what is the probability that endorses anyone during the primary campaign? There are certainly costs to doing an endorsement, as all of the candidates will want to suck up to him and pay lip service to his issues as long as he's officially "neutral".
Sanders is not Obama. He actually has convictions.
If he opts not to run (remember, draft movements are a thing) he will most definitely endorse and, for better or worse, will probably do so early on, just as he did with Keith Ellison.
I guess what I mean is, even if Sanders is only concerned with advancing his issues rather than his own ego or political career, there is a logic to not endorsing. By not endorsing, the other candidates compete for him and his voters, which means embracing his issues, thus helping his agenda.