1. A web app developed using WebAssembly offers performance indistinguishable from that of a native app.
2. Since the web app offers native performance, there's practically no need for a native app to exist in the first place. Why should developers waste their time developing for multiple platforms when a single mobile app works flawlessly?
3. This is great for users because the app is highly performant, universally accessible, and doesn't require installation.
If the performance of your app is limited by the DOM, this will make no difference. Most applications are not doing heavily calculations, they're animating an non-native HTML UI so WebAssembly will make no difference. Raw calculation performance is not the biggest issue for mobile apps.
> Why should developers waste their time developing for multiple platforms when a single mobile app works flawlessly?
Nothing about WebAssembly will make web apps work "more flawlessly" than web apps work right now.
Since WebAssembly doesn't change the user experience of acquiring and using applications it will make no difference, from today, on adoption on web applications.
I'd go so far as to say that users actually don't like going into Mobile Safari run mobile applications and WebAssembly isn't going to change that.