A few points:
1) There are younger folks who are
also minorities (and he even
won those
groups in various categories). It's actually much more accurate to say that Sanders didn't get enough support from
old people, but it's interesting that the narrative was never "Sanders had an older voter problem".
2) A preference for Clinton and/or her policies (especially if someone agreed with the "electability" and "realism" argument at the time) is not the same as a dislike of Sanders and/or his policies. From the
previous link again, Sanders favorables remained high across all groups, and as far as I can tell, has still remained high.
3)
Sanders was attacked plenty of times.
Sometimes within the same day. Sure, maybe there's some super duper secret scandal that would completely ruin him, but it's also possible (and even more likely, since he's been in elected office for 30+ years with both Democrats and Republicans gunning for him) that he just doesn't have a lot of skeletons in his closet comparatively speaking, and people would actually like him more as he becomes more of a national figure.
That view actually does fit with the evidence of his mostly high favorables over the past year compared to when people knew less about him. Available evidence shows that the more people find out about Sanders, the more they like him. "If people find out more about Sanders, they would turn on him!" is not backed up by any available evidence. It's certainly
possible, of course, but it's a much weaker claim. It's also a claim that Clinton made about Obama in 2008 that didn't pan out then either.