Worst case scenario, we can sandbox the whole browser and detect ads visually.

No, stopping ad blockers would be trivial today if the ad distributors let their JavaScript and ads be distributed from the same domain as page content. As long as the ads come from clearly identifiable domains, there is a good chance at stopping them.

If WebAssembly is primarily used for ad tech, it may be the death knell for WebAssembly.

Why? What special about JavaScript that makes this kind of analysis possible? Or what is it about WebAssembly in particular that prevents it?

How? Ads will still be served from 3rd-party domains which will be blocked, WebAssembly does not change anything to that.

Assembly never stopped anti-virus suites. Where there's a will, there will always be a way.

Without adblock, the majority of web users would be FURIOUS with websites and they'd demand legislative action. Advertisers should shut up and pray they don't poke the hornets nest.