If he runs 3rd party, it ensure a Trump victory - if he doesn't run, Trump will win anyway.
Here is a different view:
If Bloomberg runs 3rd party, he is going to siphon off those repub Never Trumpers that couldn't vote for Warren. Not a big problem because Dems don't need their votes to win. Dems just need them to not vote for Trump.
If he runs 3rd party, he will siphon off some moderate dems that couldn't bring themselves to vote for Warren. However, I think that the number of moderate dems voting for Bloomberg will be very small; to small to effect the outcome because the VAST majority of dems want Trump out of the WH.
If he doesn't run, Never Trumpers will either not vote or hold their noses and vote for Warren. With the Never Trumper votes unavailable to Trump, Warren will win in battleground states because Trump's margin of victory in those states was razor thin in 2016.
The challenge for dems is to get equal or better turn out amongst their base relative to their 2016 turn out performance. Since dems do a sorry job of speaking to the specific issues facing people of color during campaigns, the only way of turning them out in large numbers is to have a person of color on the ticket.
Either way, there is a populist movement of the left; just as there is on the right. The movement on the right elected Trump and the movement on the left will push Warren or Sanders to the top of the dem ticket. Populists on the left and right have a lot in common wrt financial issues. The system is rigged. Moderates know this and will do all they can to preserve the status quo.