What your argument is describing isn't populism in general. Trump's populism is christian nationalism which seems to be what your argument is referring to. Bernie's platform in 2016 and 2020 was a populist platform that was intended to forward a progressive agenda.

https://www.wordnik.com/words/populism

A political philosophy supporting the rights and power of the people in their struggle against the privileged elite.

Why would we discard it later? Populism isn't inherently radical or reactionary as Bernie and Trump have demonstrated. It provides a narrative to contextualize what the campaign is for to the people. Which Democrats definitely needed in this election. It's highly unlikely Democrats will make their own populist campaign. We need to hijack the Democratic Party the way Bernie tried to do and Trump did to the Republican Party.

The strategy is to use the Democratic Party so we don't have to build our own party from scratch. It's not because we like or think highly of Democrats. Because of our first-past-the-post voting system we end up in a two-party system. The Democrats are the tool we have to use when the Republicans are fascists that want to kill us.

https://www.youtube.com/watch?v=s7tWHJfhiyo