This does seem to assume that ad networks are the main or only way to support sites that rely on ads for their funding. I'm not sure why we should make that assumption, though.

We're talking about a built-in feature to safely and transparently include ad content from a third party in your own site. It seems possible that advertisers could set up standardised content to work with that feature. Then the kinds of hosts who make enough from advertising to be worth keeping it anyway could either delegate to an advertiser's own server or self-host. Payment could be either a direct transfer or use some new, simpler and more transparent service just to handle that aspect.