I can buy the premise that central democrats will largely go with a democrat socialist candidate, but will the independents? You raised a good question. Bernie is likable enough that maybe he'll do well against a polarizing figure like Trump.
The guy may be an outlier but I watched an interview of a former Trump supporter who is now loudly backing Bernie. I've heard quite a few anecdotal stories about people who only voted for Trump because Clinton was nominated over Bernie, so I don't think he's alone.
The assumption that Independents are all somehow ideologically between the two establishment parties is quite simply belied by the facts on the ground. Many just don't bother voting because neither candidate speaks to their interests- but they WILL make the effort for someone who does.
I think there is an enormous groundswell of support for the Progressive or Social Democrat Left. How big it is remains to be seen.
One thing is for certain; the establishment Democratic Party- not to mention their biggest boosters here on this forum- clearly see us as the biggest threat to their agenda. There's no other way to explain the extreme level of vitriol against one of the frontrunners in their very own nomination process.