but someone with a voice said “Bernie is unelectable” and that was all it took, his numbers dropped overnight

That is not at all what happened. I remember. I was there. The DNC cheated over and over to sabotage him and eventually just had to nominate someone who didn’t win their primaries (Clinton). There was even a court case where they admitted as much. Because their base was so pissed.

Wilding v. DNC

They have memory holed the hell out of this case. Last time I looked you could only find the transcript on the Internet archive