Social democracy isn't some "evolution" of capitalism, it was a compromise position devised to placate workers and prevent them from going full on Marxist. As there has never been much of a threat of that in the US, there is virtually no chance of that happening here any time soon, particularly given how even a milquetoast moderate like Bernie Sanders was demonized as if he was the next coming of Stalin. We have a system that puts all the power in the hands of the wealthy, and we expect that asking them nicely to give some of that up to help the rest of us is actually going to work.