The funny thing is that $15 million dollars is not a lot of money for some people. There are a hundred billionaires living in New York City and if $15 million could “buy an election” then at least a few of them would be doing it all the time. They don’t do that because it doesn’t work.
Bowman lost because he was genuinely unpopular in his own district. Being a leftist celebrity didn’t help him much since most of the people he was popular with weren’t actually eligible to vote for him.
Part of it was the money, and yes part of it was that there’s been redistricting across NY state since he was last elected, his constituancy is now clearly more wealthy, right-wing and racist.
Democrats are working hard to push the party further to the right using the same kind of gerrymandering tactics that the GOP uses to capture whole states. Dems redistrict to keep things blue, but water down the voting power of the center left.