Even worse, besides spam domains there are countless legit enterprises that like to be at the top of results that create bad incentives.

Imagine the community manages to set up a walled off non-commercial web that gains enough popularity to be interesting to advertisers. Who would be in charge of such a thing? And what would they do when Coca Cola showed up at the front door with ten million dollars in a briefcase? Federated would not be much better, they just need to pay the most influential nodes.