If you’re a democratic socialist, do you really want Sanders to be the nominee? The question isn’t as dumb as you think.
America is just not ready for socialism. If Sanders actually does get the nomination, he is likely to get slaughtered in November, and the cause will be set back decades. If, somehow, he wins, his program won’t enjoy majority support in his own party, let alone Congress as a whole. Intense frustration will set in, and the Democratic Party will tear itself apart, which only aids Republicans.
In short, he would be the American version of Ramsay McDonald. That doesn’t sound great to me. The better alternative, from the socialist perspective, is a moderate who loses to Trump, who then drives America further to the left with four more years of madness. AOC would be waiting in the wings, and lots of reactionary boomers will have died in the interim. The future would be much brighter, assuming, of course, that we all survive the four more years of Trump, which cannot be taken for granted.