Here's the thing: The traditional / establishment democrats have blindly believed that you can negotiate with people like Trump, because of decorum. Establishment candidates have been more occupied with taking the moral high-road, even if that resulted in loss after loss.
There's this ingrained belief that if you just play nice with republicans, they'll play nice back. And that was valid for a long, long time. The problem is that Trump completely upended that, though he was not the start of it - he's just the natural consequence of populism and the ground that had been fertilized for years.
The reason why Trump has been allowed to hijack the republican party, is because of how the two-party system in the US works. Trump has an iron grip on a sizable number of voters, and thus the GOP had to kowtow to Trump.
Luckily Trump is MAGA, and MAGA is Trump. I fully expect that the current MAGA movement will fracture and splinter into smaller factions once Trump is out. And from the looks of it, they do not have any plan B. No front runner in sight.