The long term problem is that we don't have a political economy that actually represents the public.
But the short term problem is that Trump is currently on track to win, and the people who don't want that to happen are sticking their heads in the sand.
We need to (1) reengage the Democratic base. Biden's victory in key swing states -- Pennsylvania, Michigan, and Georgia in particular -- was built on the backs of grass-roots door knocking campaigns by Bernie supporters. His campaign was absolutely reliant on the support of people who didn't really like HIM, but really wanted to get Trump out of office. Those people will probably still hold their nose and vote for him, but that turnout operation is shattered, and I don't see a way he can match his close victory in 2020 without it.
If progressives find a champion in Jill Stein, it's possible that they start dreaming of something better, and if Biden turns things around, they'll have the drive to rebuild that critical lefty turnout machine.
(2) Biden needs a metaphorical slap in the face. He won last time because Bernie's team wrote half his platform in a reconciliation committee. This time, there was no primary, so Biden has reverted to all his instincts, and they are TERRIBLE. He's trying to win Haley voters as if that's not like Charlie Brown trying to kick Lucy's football. If Stein gets momentum, maybe it'll knock sense into him.
This is all aside from the fact that voting Green builds party infrastructure and ballot access for a meaningful third party. There are lots of complicated reasons why voting Green has long-term strategic benefits, but I'm not even getting into those. I'm just talking about how we save Biden from himself. Sorry if it sounds like 4D chess, but polls already show Biden losing and he's not taking note, so I think seeing popularity for a left alternative is the only thing I can think of that will rescue this thing.