Alternative take: A choice between a "true progressive" and a neo-nazi loving Republican would result in the Republican win.
I'm genuinely sympathetic - I'm 42 years old, and yet I can honestly see that before I die the ice-caps will be irrecoverably damaged, the South American rainforests will be destroyed, and water will become a scarce resource.
And yet I look at the UK, where Corbyn repeatedly refused to step-down, and think "Fuck me, yes, incrementalism would mean that there wouldn't be a sub-class of people who have to choose between food and heating, and where legitimate British citizens were deported to a country they have never seen in their lives". If you ask me if I would rather have an incrementalist Centre-Left/Left leader in power, or a Left/Socialist leader forever tilting at windmills, then I would say I'm not heartless, and would rather the lives of some of the most vulnerable were improved than a right-wing government continue to shit on the poors whilst making money.
Once again, I understand where you're coming from, but you have to see that "Bernie is the only one who can save us" 1) only works if a voting majority believes that, and 2) dismisses the needs of the many until that one moment when the stars align and everything can be improved for everyone.