This has nothing to do with bytecode and everything to do with the security models used by Android and iOS.
Ad blockers don’t work by inspecting the source code of YouTube’s JS and modifying it. Instead they inspect the network requests being sent, cookies being stored, elements being created and with which styles, and so on.
WebAssembly changes none of those things.
I think the population that consumes YouTube in general is much larger than the population that consumes paid content with DRM, and there would be a much larger pushback if webpages like YouTube started to be distributed in compiled format.