I think that "look how well racist populism just did! If we run on socialist populism we'll do so well! After all our message is so much better!" is really misguided. It just...it just seems to ignore a lot of realities about the American electorate
This is only true if you only examine right wing populism from a racial perspective. That's a major element, but there is obviously more too it than just that. At the bare minimum the obsession with language about corruption is indicative of that.
Populism is heavily rural in nature- it's basically people seeing the world as though the Hunger Games is real life. (man, that movie looks outright poisonous in hindsight.)
It's also majoritarian, which causes issues w/ minority groups.
Maybe in 21st century America. Before that I think you'll find it's actually heavily urban. Especially in the 19th century.
The decline of the social democrats is understandable: on the one hand they reacted to Thatcherism and its equivalents by tacking towards neoliberalism, eroding some of their base's enthusiasm. On the other hand, the success of capitalism after the Treinte Gloriouses had diminished the appeal of socially-conscious parties.
The New Gilded Age is bringing back the perceived need for hard-left parties, when that need never really vanished.
I think we can flesh this out a lot more, but you are more or less on the mark. Something changed in the late 70s and 80s and it's been disastrous for politics.