Ross Perot and Ralph Nader both got significant percentages of the vote. Both instances had the same outcome: they caused the major party nominees most closely aligned to them to lose, skewing policy even further from their position.
Bernie having given Hillary a close race was able to change the Democrat platform; had he run third party he would have caused her to lose the general election and policy would become very conservative. Do progressive Bernie supporters want conservative policy? Of course not, but based on past elections that is most likely what they would get.
I’m nor saying don’t vote third party; I did last time and I may this time. It is an easy though relatively ineffective way to voice dissent in a non-swing state. Swing states are much different since every major party vote can effect the outcome.