I don't think Bernie would have won either
For my money, he's the only person who could have won the centre. Rhetoric and oration is so important nowadays in American politics and he can stand toe to toe with Trump in a credible way. He'd take the Democratic left along with him but when he talks, he talks to people in the Rust Belt about how a sea change could positively affect their lives.
And we shouldn't ignore the fact that he's a man. Seems nasty to say, but it appears from the outside that that centre ground has a sexist aspect to it whereby they wouldn't trust a woman to be strong against Putin or threats whereby the masculine stereotypes would play into Sanders hands.
It's also worth pointing out that if you look at the modern Western world, people are voting for "change" above all else. We've had the generation of slick politicians in the late 90s/00s who ooze PR and a big part of the Trump vote was about appointing somebody who was offering a completely different voice to the normal. Sanders did the same on the left which is why his candidacy was so energised.
The Dems made the same mistake that Labour did - they saw the groundswell of energy behind a candidate then attempted to cut him off rather than roll with it. Ultimately Labour killed Corbyn by not jumping behind him lock stock when that support from the grassroots became clear and it has led to him being an ineffective leader who now won't win an election. Sanders is still untainted in that he hasn't been the top dog so the sniping from his own Party wouldn't affect him in the same way. If they got behind him and pushed him then he's an ideal middle candidate and as I say, he's someone that Trump has had difficulty in dealing with before.
In the eyes of the swing Trump base, Warren is already a dead property due to the Native American thing, the woman thing, and the SJW thing. Sanders supports many of the same principles but can go for Trump in an oratory fist fight and isn't tarred with that brush to the same degree.
It's worth noting something that Trump said recently and I think is illuminating to his ideas. He spoke about how he should stop talking about the Warrens of the world because he's going to end up knocking out the easier candidates. This could tell you that it's the people who are high profile but he doesn't really mention as much that he actually fears - he understands the power of getting the narrative right against potential enemies in the opposite Party and like he did with Clinton, he's built one against Warren. Sanders is something I think he's worried about. Worth noting also that various Republicans actually WANT Warren to run. That's a bad sign for her.
A Sanders-Warren ticket could be a good option. Sanders-Booker is a bit uninclusive for the left of the Party who want a woman. Booker-Warren is a bit lesser than Warren-Sanders and Warren-Booker is going to drop dead if they were ever foolish enough to try it. People have mentioned Clinton again which is a bit of a poisoned chalice if she ends up on a ticket. Clinton-Biden or Biden-Clinton, if it were ever to go in that direction which is unrealistic obviously, may as well shoot themselves in the head. Sanders-Biden is a bit "old white man" for the Dem left and Trump could play on the age thing (despite being older than both IIRC).
Delaney is so boring that I fell asleep half way through typing his name. He's John Major with an accent. Avenatti is a joke candidate and would landslide the Republicans. John Kerry would have a real shot at winning but I get the feeling that he's only playing games here and has no real intention of running. Bloomberg won't get the backing from the Dem left. Harris will be President in the future if she has the patience and is probably the future of the Party, assuming the gun lobby doesn't get to her first, but Trump would savage her on abortion/gun control/immigration. She's the person after the person if that makes sense.
2020 is one of those races that the Dems will win if they pick the right candidate. I just don't think they have it in them to pick the right candidate.