I think electability is in theory an important consideration. You seem to give it short shrift by optimistically saying all the Democratic Party candidates can beat Trump.
I’m sure that even though we cannot underestimate the challenge, virtually everyone currently running to be on the democratic ticket has the power to beat Donald Trump.
You say “we cannot underestimate the challenge” and then you go right off to underestimate it. My prediction is that any of the coastal Progressives will be decisively defeated in the electoral college and will also lose the House of Representatives in the process. Mix in impeachment, a credible third party centrist, and a strong economy in November 2020 and you have the makings of a massacre.
What your analysis needs is some data or even informed speculation on state by state electoral votes. Can a given candidate keep all the states won by Clinton and grab a few that Trump won? Which states will flip and why? To put it another way, it is unrealistic wishful thinking to assume any Democrat can win.