MS Flight Sim might be a good pilot case (pun intended) for WebGL and WebAssembly. The nature of simulator is not that CPU/GPU taxing. I would assume the players play it more for the actual simulation and less for the specular graphics.
I think you're underestimating just how much simulation is being done in a proper flight simulator. The really good ones are literally simulating air flow around the aircraft, in realtime, to determine how it should respond.