I'm proud I didn't vote Blue in 2016 and 2020. I'm not from the USA so it would be a voter fraud if I did.
That being said, dems are completely incompetent when it comes to elections, deliberately picking such terrible candidates like Hillary and Biden. I don't believe there are actual people that voted Blue because of seeded candidate and not despite it.
Not to excuse the Democratic Party, but I would argue that it's inevitable that they choose shit candidates for the presidency when we consider how much indoctrination Americans get over their capitalism/corporatism and how they justify and normalize the shit America did and the status quo, while omitting a lot of the important progressive historical events or (in the case of figures like MLK and Gandhi, or context behind Iran/Latin America/Israel&Palestine) don't tell the full story. Like most Americans aren't going to learn that the US denied millions of Jews, Roma, and other groups being displaced entry into the country right before the holocaust, leading to the deaths of millions. You also won't learn that the US installed multiple fascist dictators, monarchs, etc. in countries after overthrowing democratic governments to serve corporate intrests or over their fear of leftist leaders. At best you might briefly touch on the wars with Iraq in government class or something.
Then you're basically indoctrinated by history classes and society that anything left of American corporatism is communism. To the point that modern conservatives genuinely think FDR policies are extreme socialist/communist policies.
With our current voting system, an actually social democratic president would not get elected. FDR, Truman, Johnson, and Carter were the closest we could get in that regard, but those days are long gone.
That being said, I do think a lot of people genuinely voted for Hillary because of her and not solely because of Trump. Especially women and LGBTQ who wanted like... more gender equality. I think more accurately would be that there are very few white guys who voted for Hillary for any reason other than Trump being worse.
I agree with most of your points except with one about socialists being unelectable - I believe that Bernie would have moped the floor with Trump in 2016, and even more so in 2020. First woman president had a nice ring to it, but Hillary performed terribly, and with her being basicially an aristocrat of the establishment, lots of people hated her from the get go. They shanked Bernie so their corporate overlord donors wouldn't get pissy, and voters treated that as betrayal. After very solid 8 years of Obama it get like Dems had this victory in their pocket regardless of what they do, and they commited to proving that hunch wrong. That's at least the vibes I got from watching the elections from Europe, I did hear that Obama wasn't as popular as he seemed to be.