Three reasons this is incorrect:
1) Bernie will draw millions of non-regular voters to the polls (like Trump did)
2) While Trumps 46% is locked in...
3) Democrats mostly like Bernie and will rally around him if he's the nominee. The party has moved so far to the left that they don't view Bernie as extreme.
But if I were to guess who the weakest DEM would be against Trump and the one DEM candidate for whom there could be a "Never _____" movement, I would go with Bloomberg.