1. Publishers would shift to apps. Apps won't support ad blockers, and are in a much stronger position to detect them and refuse to operate until the block is removed. I'm not sure how much of this would be the "one big app" model where you use Facebook/Amazon/Reddit/Google for everything vs a "lots of little apps" model, but I'm guessing more likely the former?
2. The ad blocking arms race would accelerate dramatically. Sites would CNAME to their ad network, which would then be responsible for proxying the site and inserting ads directly into the pages. The ad network would try to change its serving approach faster than the blockers could keep up. This would look a lot like how Facebook runs ads on their site, with obfuscation to confuse blockers.
Fundamentally, publishers are offering a trade: we're willing to show you our pages if you'll view our ads. Users with ad blockers are rejecting half of the offer and taking the other half. Publishers can't force users to see the ads, but they probably will be able to use technical means to keep users from getting the content without the ads.
(Disclosure: I work for Google on ads, speaking only for myself.)