Again, based on what evidence? If you ask about specific Sanders' policies, routinely more than half of Americans agree with them (like universal healthcare, green new deal, economic support in dealing with COVID,...). So I think this new platform could have large support - except people are worried that nobody would vote for it. (But the problems with American voting system go far beyond that - voter suppression, gerrymandering, delegates, bribery of the supreme court; the whole system can only be considered democratic under a very antiquated definition.)

In the 2nd sentence, you're assuming exactly the conditions of the two party system. In Europe, new parties (like Greens or Pirates) were successfully able to bring new issues to the traditional political divide.