The group that Trump used to flip the upper midwest was non-college educated white guys. Is Bloomberg going to appeal to them? Trump doesn't have the policies to appeal to that group but he has the personality to appeal to them. I'm not certain Bloomberg does.
He probably appeals to some Biden voters, but not all. I don't see Bloomberg with the same appeal to minority voters or working class voters. I'm also not convinced there is a Democrat candidate out there that will actually sway any fringe Republicans or right leaning independents.
I don't think left-leaning is as bad as some think. I think it energizes folks who won't be excited by Bloomberg or Biden. Think Bernie voters who sat out 2016.
Bloomberg is old too. It would be nice to have some younger blood in the thick of things.
All that said from my perspective, I could definitely see a lot of Democratic primary voters thinking he's a less senile but just as electable version of Biden.