I know this is an impossible question, the future is chaos, but we can at least identify and discuss some key factors.
When I say "successful leftist revolution" I mean one that happens soon enough to prevent, or have non-negligible odds of preventing, the collapse of human civilization as we know it due to climate change.
In all likelihood I think we're fucked as far as climate change is concerned, but we should still try to improve our odds. There's no prize for correctly predicting that we're fucked.
Why would Joe Biden winning devestate the left?? Everyone on the left knows that electoralism and the DNC suck. Bernie grew strong under obama.
Some people are so wrapped up in justifying their votes/non-votes that they ignore history.
I'm not denying that the left is in one of the best positions it's been in for years. However, I think a lot of these ideas we're championing got a lot of headway with mainstream libs because of Trump, or, rather, as a reaction against Trump and an increasingly deranged capitalist class. Will these libs still care about these issues once another corporate Dem takes office? I'm inclined to think not because history shows libs don't hold their own accountable, especially the corporate wing. I hope I'm wrong, honest.