I think he has a history of "walking the walk" more than anybody currently officially running, which voters (only sometimes I guess) care to keep in mind. I don't think any of the current candidates making a mad dash to the left from 2016 to now necessarily have that huge shift as an advantage when he's been there all along. But you're right that he stands out less as a result.
That's my appeal. He has been on the right side of history and has walked the walk all this time. Others are coming on board, but why go with someone that may be wishy washy to appease the current crowd when I can go with the OG.
I do think AOC will hold off on endorsement for the longest but it would easily go to Bernie if she had to choose from the possible candidates. I imagine the next orders would be Warren and then Harris.