If the canvas website really was a threat we would have seen much more of it by now already, but it hasn't happened for a number of reasons -- we only see them in the dedicated desktop-app-like use cases like this one. Anything scrollable won't be viable because it would be inherently less responsive than the real deal. Fonts look different, most designers actually prefer for people to be able to select text, and be able to use browser functionality that looks native for the different OSs. Canvas websites are also more expensive in terms of blob loading and ability to cache pages. So it's not really a threat.
Most adblocking happens by the means of network/URL blocking and I don't see any potential difference between WASM and JS in this regard(unless the APIs are enshittified, but JS isn't impervious to this too). If ads can't be blocked by network, and the website proxies ad contents through its own domain(as far as I know that's not prevalent, maybe I'm ignorant -- they need to track users and clicks themselves anyway), than you could get hard to block websites with JS+DOM too -- it's possible disrupt DOM based blocking by frequent randomization/obfuscation to the degree that one would need some kind of advanced AI to even hope to find the ad DOM elements. Between WEI, web bundles, Topics API push, webassembly's supposed higher indecipherability (which isn't true, chrome "disassembles" WASM into the WAT form and it's pretty readable. More readable than wasm.js) doesn't even register.
>And for what?
To not deal with JavaScript? To be able to develop for the web in the language you want, without floats being the fundamental numeric type and other BS? Is that too fantastical of a desire?