Sorry
@NinjaCow64 for taking so long to respond to your thoughtful post. I was not ignoring you and meant no offense:
It is my opinion that you are peddling a revisionist recent history of the Democratic and Republican parties and you are blaming Democrat voters for the failures of the Democratic Party machine.
If the Democratic party is failing, its the fault of Democrats. "The Democratic Party machine" is just a bogeyman. Its a way of shifting blame from Democrats, ie the Democratic voters, to some faceless "they", in order to absolve the voters of any responsibility.
This is just not true. When the Republicans ran Republican Bidens (they did it twice, both with McCain and Romney) and both of them failed to resonate with Republicans and voters in general. Republicans DID lose heart and #abandon the party until someone who they actually thought would deliver them what they wanted, Trump, arrived.
People also commonly forget that Obama ran on a platform of Hope and Change. What he did in office was a sharp betrayal of what he promised voters, but Obama's success was a clear indication that Americans (Republican, Democrat and otherwise) did want change.
Multiple things wrong here. First, you seem to be contradicting yourself here. On the one hand you are saying that the Republicans lost because they ran Biden-like candidates, then you are saying that they lost because the Democrats ran a Trump like candidate. Which is it? Did Republicans snatch defeat from the jaws of victory by running establishment candidates, or did they get steamrolled by an unbeatable generational messiah candidate? While its certainly possible to say both factors contributed, to make a valid argument here, you've got to take a position on which factor was the most important.
Second, your premise is flawed. McCain's brand/appeal was not at all like Biden. McCain's whole deal was that he was a "maverick", ie he was promising to buck the Republican establishment and deliver change, immigration policy being one prominent example. Palin as his running mate was an extension of this as she would have been the first woman to be VPOTUS, and she had cultivated a "maverick" reputation herself. In fact it was McCain's divergence from the party line that made many Republicans unenthusiastic about him. He didn't hate illegal immigrants enough and he was too civil/gracious towards Obama for example.
In any case, I think some of your point(s) have merit. Romney was certainly a consensus candidate that Republicans weren't as enthusiastic about, similar to Biden on the Democratic side... but consider that
Romney lost that election, whereas as
@Lexicus points out, Biden won. So the point you seem to be making does not seem to stand up. A better analogy I think would be Hillary, who was certainly the non-negotiable establishment candidate, somewhat similar to Romney in that regard, and she lost to the "change" messiah Trump. Remember that Hillary also lost to Obama in 2008.
Its also the case that both McCain and Romney received substantially less votes than Baby Bush in 2004, still enough to beat Kerry, but not Obama. Its also worth noting that Hillary got less votes in 2016 than Obama in 2012. I think what all that points to, is more about the strength of Obama as a candidate than the weakness of McCain, Romney or Hillary, especially since Hillary actually got more votes than Trump (5th most in US history) despite her losing in the EC. Trump got the second highest vote total in US history in 2020, second only to Biden's vote total in 2020 so again, the elections seem to be more about the strength of the candidates than the weakness, ie "establishment" nature, of the opponent.
One of the criticisms of Democrats that I've heard IIRC recently from
@Gorbles , is that (paraphrasing) Democrats should stop over-promising. But there is tension between that, and what you seem to be, correctly IMO, identifying, specifically, that part of what made Obama such a strong candidate was all that soaring pie-in-the-sky that he was promising, but ultimately, failed to deliver on. Do you see the catch-22? Anyway, the bottom line is... the Democrats don't lose based on whether they run establishment candidates. They lose based on whether their voters turn out and vote or not. They've won with "establishment"/consensus candidates and they've won with "change" candidates.