Not sure in what world nearly 50% of the vote is "a little band". Besides, Biden won't get a majority either, it remains to be seen if he will get a plurality and by what tiny margin.
This is only a fraction of the Democrat party voting.
In the US they're heavily concentrated on the coasts. That's not where the election needs to be won. 2018 midterms the progressives underperformed. The swing against Trump was in the suburbs where centrist candidates carried the day.
The Democratic party is a rough coalition of 3-4 parties or factions. Each faction is maybe 10-12% of the overall electorate.
So the Uber progressives at the most would be maybe 20% of the electorate. If they're hardline about it and don't compromise they can't win.
Let's just say Bernie gets 40% and Biden gets less and Warren and Tulsi carry 30%. The DNC decide and super delegates decide to support Sanders.
He goes on to win the general election. Yay America is socialist. However the other 60% of the Democratic party don't have to support his policies and they answer to their electorates not the president and not the progressive wing of the Democrat party.
Assuming Sanders has the Senate as well to pass any laws he has to get the majority of the other 50% on board.
Even if he gets 60% instead of 40% that's still enough to torpedo anything he wants to do. So he has to compromise.
If a president is super popular or has enough "political capital" to convince the rest of the party to fall in line they have to work out where they can spend that political capital. It's generally enough to pass 1 and probably no more than 2 pieces of significant legislation. Trump managed to pass 1 with a compliant party.
The main issue in these elections seems to be healthcare. Not free tertiary, not global warming, not LBGTQ rights.
The other problem with populists us they promise the moon, sometimes get elected and then discover they can't deliver. Which often gets them voted out.
You've only got two years to pass legislation and then it's midterms. Lose control like Trump did in 2018, no laws passed.