I don't think it is necessary to make websites extremely complex, to prevent you from blocking ads.

I think blocking ads is a game you can never win. If you are able to block ads on some websites today, it is just because the site owners allow you to do so (they could easily prevent you from doing so). Ads technologies can evolve to a point when you as a human can not distinguish between ads and the rest of the content.

It would make content control more complicated, but it can't make it impossible.

If the embedded browser uses the host's HTTP implementation, then you can filter and edit content there.

If it implements HTTP itself on top of raw sockets, then you can intercept traffic on socket level (although you'd need to MITM any SSL connection).

If the inner browser has its own client certificate or simply checks for a set of known server certificates, MITM won't get you anywhere. Done right, this could well be game over.

For it to be truly game over, the wasm implementation it's running on must also protect the wasm payload itself, otherwise it would be trivial to patch that to MITM it.

So, on something like iOS, it could be game over (but I'm also curious what Apple is going to do with wasm there, given that it goes contrary to their strategy of being the gatekeeper for all code running on the platform). But not for the rest.

Adding a ton of obfuscation will do the trick. Patching wasm requires reverse engineering and that is very cumbersome. For Blu-Ray it took months to reverse engineer the copy protection VM and that was with a mechanism that took a lot of attention. If it's your average website, it's far easier to stay ahead of the game.