Wouldn't there be a point where the mainstream approach is to just have compiled blobs write directly to the canvas, say a point where someone compiles a qt application and hosts it on their website.

What can you even modify there, when all the structure is flattened into a single layer

You can already make apps that write directly to a canvas today, and almost nobody does because they want what the DOM provides for them. Wasm changes nothing about the APIs available to web apps.