Fine. I concede: "I think this may happen."
My reasoning: Biden is completely unpalatable to the re-ignited left and young voters. They see him as old and out of touch. They literally view him as a conservative politician. If Biden wins the nomination, it will take the wind out of the sails of progressive voters. It will extinguish their excitement for the next election and they'll be less likely to show up to the polls.
I think the progressive wing of the Democrats and young voters are not as important to US elections as we would think. Much of the media is run by urban liberals, often younger, out of liberal urban big cities in Democrat-heavy states ie: LA and NYC. Just look at the late night talk shows. Twitter is also not a fair representation of how Americans think, it's dominated by younger people, my parents don't tweet. Gary the mechanic and Flo the Walmart cashier don't tweet.
Meanwhile, people 18 to 29 made up only 13% of the US electorate last time:
https://www.people-press.org/2018/08/09/an-examination-of-the-2016-electorate-based-on-validated-voters/Non-college grads make up a healthy majority of US voters, 63% in 2016. University students are very vocal activists and support people like AOC and Bernie and dominate Youtube comment sections, while your average working stiff is busy raising kids. Your average working stiff is who decides elections, which is why Biden has been leading in the Democrat polls. Key swing states like Ohio, Michigan, Pennsylvania, Iowa don't scream progressive Green New Deal supporter.
The Democrats need to capture the swing voters in swing states. That means moderate voters. If they nominate Biden, I think he might lose because ya he's old and seems semi-senile and has also never been a great candidate, even when he ran against Obama and Hillary in 2008.