Once the New Deal coalition fell apart after the Civil Rights Act passed and the GOP implemented the Southern Strategy and right-to-work laws destroyed unions, the Democrats lost unions as their base of financial support. They still held on in Congress for a while, but couldn't win the Presidency until the Third Way/neoliberal Democrats and groups like the Blue Dogs took charge of the party. At that point, the Democrats became captured (at least financially) by Wall Street and rich urban professionals who were socially liberal but fiscally conservative. There's no "firing" Clinton at that point. The party has been captured. The Democrats are also a much more diverse party since they are as much "not-GOP" as anything else. Why should a conservative Black voter vote Democratic? Of course they shouldn't, but the GOP was pretty explicitly home to racists for decades.

Both parties continue to self-sort in unhealthy ways. The GOP needs more diversity (but of course, is doubling-down on gerrymandering) and the Democrats, frankly, need less. I'd love to trade off some fiscally conservative Democrats for some working-collar populist Republicans. Bernie's support came from appealing to what used to be the base of the Democratic party: the working-class. But it's not enough voters in the raucous big-tent Democratic party to win a primary, and likely not the general election.

We also can't even have a serious conversation about policy since the GOP attacks everything the Democrats do as socialism.

Anyway, to your last point: the left is part of the Democratic party, but it doesn't have a strong-enough hand to play.

Also, look, voters are weird. I volunteered as a Democratic poll greeter here in NC with two women, both professionals, a white woman originally from Cincinnati and a Black woman originally from New Jersey. They'd both voted for Biden in the primary. I was like, what, why? Why didn't you pick someone more progressive? They both said: "because Biden is who we thought would make the strongest general election candidate." So they're not even voting on who they think is best, but who they think the rest of America will accept.

p.s. don't be so hard on Hillary. Let's not forget that she expended a lot of political capital trying to pass universal healthcare. She got killed on that, by Democrats and Republicans alike. I think that really changed her idea of what was possible in politics in the US. She is not the devil she's been made out to be.