When we talk "extremist".... wouldn't FDR be considered one using the "let's not get TOO radical" treatment?
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Look to where the Republicans are, where they want us to go. Certainly not forward, with their flawed, reflexive "market" solutions which would see a rise of "skinny" health care plans with reduced coverage.
First, I want to repeat that I am closer to socialist than democrat. I am a registered independent and have been for some time. Obviously, with only a two party system at the national level, that doesn't leave reformers much choice.
There are many things that I do champion far to the left: universal health care, either nationalization of big pharma or removal of barriers from importation, minimum wage at $15 or higher, rent control, government subsidized housing, heavy taxation of internet giants, fossil fuels, and Wall St. All that said, the migration must be metered.
Roosevelt was a rare case. He was extremist only in so much as Jimmy Carter was, elected during a time of broad national crisis. FDR indeed tried many radical solutions when compared to anything previous in the US, including some illegal actions that were not Constitutional, but we all made it through and were better for it, for a while. I believe we are in national crisis today, but I don't think my view is that of the majority of the nation. They view us as economically ok, and the rest is whatever, per Clinton's "it's the economy, stupid!"
Unfortunately, Biden is capable but more and more each day appearing caricature-like in his debates, resembling Walter on Jeff Dunham's lap.
I fear it won't be just the Yangists who sit at home once the far left loses in the final convention, and by that time Trump will be acquitted by the Senate and appear invulnerable. That will likely cause disillusionment in all the reformers. Bloomberg or Biden, or another centrist will head the ticket. I will say this. I don't agree with the billionaire solution, but it may be possible for Bloomberg to simultaneously sell himself as a reformer due to his gun control background, eliminate the worry that Biden's misspeaks have caused, and appear centrist enough to win over the moderate GOP bunch who would like to drink Trump's blood.
He's also a New Yorker, and God knows the media thinks New York is Zion, so there's that boost.
The only hitch will be the black vote. But I think that would be handled by a savvy choice of VP. In addition to Booker, Harris, and Patrick, there are some surprise possibilities like Condoleeza Rice or others who have high name recognition. In fact, choosing a black from a GOP administration might be the selling point, but I'm off in fantasy, I admit. I'll sit down now.
Biden was used to assuage the Old Line Democrats who might have been less enthused about a relative newbie like Obama. 
