Running against Trump is different than winning over a room. Winning over a room is a personal thing, face to face, them hearing his message.
Running against Trump is running against the Republcian machine. One that will be hammering Bernie on every level, including socialism, socialized healthcare, and Bernie can have a tendency to be professorial with all of his math which is something Trump can easily exploit.
I think Bernie was a much better choice than HIllary in 2016...but in 2020 I think there may be better differentiation available. Bernie and Trump are both old. Bernie is strongest on issues dealing with the economy, less on security. Trump has had some good economic results in his term which reduces Bernie's effectiveness there, and he also isn't strong on security issues and Trump pretends to be strong there.
Would be interesting to see a breakdown of how you see Bernie stacking up against Trump on important issues. Healthcare, economy, immigration, wealth redistribution, social programs, foreign diplomacy, Russia, etc.