Politically, the hope is that the upstarts might even contribute to a revitalization of the established European social democratic parties and the Democratic Party in the US. These parties are now barely distinguishable from the conservative parties to their right and, as such, are eroding popular support for the two-party system in the Western democracies, the favoured form of capitalist rule.
Here’s one example of that line of thinking. It’s a debatable line, but not entirely fanciful, recalling how pressure from the left stimulated capitalism’s impulse for self-preservation in the 30’s.
http://www.bloombergview.com/articles/2016-04-12/the-political-center-is-shifting-to-the-left