Does the progressive left need to be more radical?

I normally bat for the progressive team on SW but my political traditions are somewhat different (though overlapping) to American liberals.

I think mainstream liberal parties across the West made a big mistake abandoning the social democratic models of the 70s. By the 90s there were only minor differences in economic policy between them and their Conservative rivals. This may have made electoral sense at the time but the long term cost has been disillusionment of their working class and or left leaning base support. When an economic and political crisis hit post 2007, they have been seen as part of a failing establishment.

That for me is the biggest reason why Hillary lost to Trump and why Brexit happened. The nationalist right offer something different and offer answers. Now these answers made be castles of sand and the sales pitch may involve an appeal to reactionary instincts but for this to have happened, the left has done something badly wrong. The rust belt should be a Democrat heartland and northern England should similary not have shot themselves in the foot by voting for a Tory Brexit.

I am aware this is not clear cut. Jeremy Corbyn is a radical but his Uk Labour party are struggling badly, though there are many factors involved.

The German SDP is having a renaissance though and is doing so by tacking to the left. When you have a united party, a capable leader and an economically progressive program, the left can succeed. Opinion polls had Sanders beating Trump by ten points during the Primary season and I think this is what the left should learn. Economic populism can stop the nationalist right.