Likelihood of winning the general election definitely matters. I am undecided as of now. I think Warren would probably be the best president, but I am not sure she is the best candidate. And even the worst in this field would be worlds better than another 4 years of Trump. So it matters. It matters big time.
Party leadership (and even primary voters) definitely deserve some of the blame. If one of my favorite teams loses, then I am generally more pissed at the coach for having a shitty strategy than at the other team. And putting up a candidate that half of the country absolutely hates (even if their reasons are stupid or immoral) is a stupid strategy.
BTW, I like Pete a lot too, but the jump from mayor to president is huge. We need a president that can undo as much trump damage as possible as quickly as possible, and experience in Washington matters for that sort of thing. Hopefully Pete can build a name for himself and then run for something more substantial.