Because an application like that is both computationally expensive and expensive to ship as a runtime. WebAssembly can provide benefits in both cases (WASM bin

Because an application like that is both computationally expensive and expensive to ship as a runtime. WebAssembly can provide benefits in both cases (WASM binaries can be close an order of magnitude smaller than equivalent JS code, in select cases), and is quick to start. As a simple example, it's not impossible that Adobe could literally take the flash VM code, compile to WASM with emscripten, and ship it along with an SWF file and a little setup code. This completely bypasses the need for a browser plugin for flash. Now, I suspect the flash VM might be large enough to make this noticeable, but it probably wouldn't be hard for Adobe to streamline it, and then re-purpose a lot of those tools used to build flash that everyone loves to rave about... I believe there are financial incentives for this to happen, therefore I think it's only a matter of time until it does.