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.
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.