Stein is mostly a gadfly, but gadflies get attention. The dems have no incentive to court leftist votes if they assume we're too small to make an impact or we'll fall in line no matter what they say. The possibility of the Green party becoming viable is one way to demonstrate that that isn't true. I prefer PSL as well, but it looks like the Greens are marginally better known and currently ahead in ballot access.

As for AOC and the DSA, the latter rescinded its endorsement of the former, so I don't think you can treat them as associated. As for why that endorsement was rescinded, it was because AOC got that power and then refused to use it to advance the DSA's objectives. She executed on a plan to gain power, sure, but she's not putting that power to good use, so I don't know what the lesson is there. It's easier to get power if you submit to the establishment?