Eboue and I have shown using a variety of evidence that he is the most popular politican in the country, and 3 years in the public eye have done nothing to significantly harm those numbers.
Polling generally got stuff right within margins of error and showed him heading to a landslide.
Honestly, I don't see any basis for your claim about limited support.
This was the latest one I've seen (March 2018 I think):
1. The HRC numbers from the same time were consistently lower than Bernie's by about 5 points. The last set of polls for him was June 5 2016,
he was +11, she was +1.5. So, Trump had much less of a mountain to climb when facing her.
2. Several of the attacks he used against her ("Crooked", "bad health", "husband is a rapist", "swamp") wouldn't have worked on him. So far, the 2 year-long low-level warfare on his twitter page about a 3rd house and a corrupt wife have moved his numbers about 2%. Yes, he would have been hit and bled a few points for being a socialist ("crazy Bernie").
3. Popularity. She was already underwater (-10/20, Trump was -20/30) from the start of 2016, he was holding steady in the
plus tens/twenties. That's the sign of a candidate who would be harder to attack.
Again, I don't think he would have actually won by that 11-point margin. Most Republicans would have as usual come out for their man. But I don't think there's any way Trump could have made up a 11 point gap, when he barely scraped past a 5-point gap versus a more vulnerable candidate. And the crucial ones - Wisconsin, Penn, Michigan - I don't see Bernie losing those.