It's not a coincidence that the two people who received the least scrutiny and attacks (Kasich and Sanders) polled the best. And the reason they received so little scrutiny/attacks is because everyone knew they had no chance at winning.
For a month or two Ben Carson had the highest favorability rating and GE numbers due to the fact that he was mostly ignored and was just seen as the nice friendly likable black doctor. Then he completely collapsed under the slightest bit of scrutiny. Sanders' image and poll numbers would take a hit if he was the nominee or looked like the presumptive nominee. The only question (which none of us will ever know) is how big the hit would've been.
I tend to agree with this. Sanders is kind of a one-pitch guy; he's the guy with a 100-plus MPH heater that doesn't really have a good breaking pitch, and he doesn't change speeds well. He can, however, control his fastball, so he gets key strikeouts. But he's not a starting pitcher in the minds of folks. Hillary Clinton is a third starter, not really a top-flight #1 starting pitcher, but she's a starter, and her ability to go 8-9 innings in a game is not in doubt.
I think Bernie hurt his campaign greatly by describing himself as a "democratic socialist". If he were the nominee, this aspect of his campaign would be the focal point of the GOP's campaign, and lots of folks would get squeamish. He should have kept his mouth shut on this issue, IMO.
