Biden decisively defeating progressive opponents was far from a fiction. In fact the opposite is true: the media wrote off Biden at the outset, and his campaign nearly ran out of money until key endorsements later in the process. Instead, the media propped up progressive candidates like Elizabeth Warren for most of 2019, who in retrospect were not remotely viable. The Democratic Party in America can win national elections only by getting a super-majority of Black voters in the south and Latino voters in the southwest. In Texas, Warren won just 7% of Latinos, and in Virginia she won just 7% of Black voters. Michael Bloomberg beat her for third among both groups. Bernie Sanders, meanwhile, did well among Latinos. But he was a distant second to Biden among Black voters. And he alienates many among the dominant part of the Democratic coalition--affluent, highly-educated white professionals.
There is a lot of anti-establishment sentiment in the U.S., but what I think outsiders don't understand is that it's fragmented between the parties. Many anti-establishment, working class people are socially conservative as well, and vote Republican. They have no place in a party that talks more about race and sexual orientation than about immigrants taking American jobs. By the same token, many progressives are social liberals first and foremost. For them, opposing second trimester abortions is a bigger sin than working on Wall Street. That divide exists to an extent even within the Democratic Party--most of the "progressives" I know hate Sanders and think he's a "racist" and "sexist" "old white man."
This means that a key progressive premise--the idea that if the party only moved sufficiently left voters would "turn out"--is a mirage. Yes, there's a lot of anti-establishment, apathetic voters out there. There was a huge surge in turnout in 2020. But out of people who voted in 2020, but not in 2018 or 2016, fully 47% voted for Trump, versus 49% for Biden: https://www.pewresearch.org/politics/2021/06/30/behind-biden.... There is just about zero chance that any of those Trump voters would have turned out for any of the progressives apart from Sanders. (But, as discussed above, Sanders couldn't carry the social liberal wing of the Democratic party that Biden carried.)