You're talking sensibly, but the people who vote in the Democratic primaries are not going to be sensible (and for that matter, I think the people who voted in the Republican primaries in 2016 were also insensible).
First of all, Hillary, the electable one, lost. Bernie, the guy with all the enthusiasm behind him, did not. Yes, he lost the battle for the nomination, but let's face it the party machinery did have its hand on the scales, and Bernie did not campaign hard against Hillary early on, probably because he did not feel he had a chance until he started winning.
And anyway, who is the safe, dependable candidate this time around? Joe Biden? I guess he's young compared to Bernie. John Kerry? I'm just not seeing anybody suited for that particular role, which may be why both Biden and Kerry have made noises about running, but I can't really see them keeping the establishment Democrats together the way Hillary could.
So it's wide open, and I have a hunch that the Democrats are really not tired of losing enough. And what I mean by that is that they can sell themselves on the idea that they should have won 2000 and 2016, and if they'd won 2000 they would have won 2004 as well, which means they'd have a winning streak of 7 consecutive presidential elections.
And in fairness, the GOP got pretty lucky in 2000 and 2016; I don't deny that those elections could have gone either way easily. But of course if Al Gore is President in 2007 when the economic meltdown starts John McCain or Mitt Romney probably waltzes to victory in 2008.
So I think the "trim the sails and get a win" strategy is going to be hooted at among the Democratic primary voters in 2020. They'll fall in love with somebody (Warren, Harris or more likely some handsome male politician). But he's going to have to be very solidly liberal and get the support of the netkooks, err, netroots.