Are there ready made engines suitable for this?
Unity has taken 11 years and Three.js 6 years to get to their current estabilished status, so let's keep fingers crossed that something gets started soon on the asm.js/emscripten front.
I think that's a bit of a chicken egg problem, the large game engines need to be everything to everybody, and trimming the executable size down probably wasn't a priority in the past since mostly you'll have a big upfront download anyway.
However, I remember that the first Unity HTML5/WebGL demos a few years ago were much smaller, a complete zombie FPS had a 3.5MB upfront download which is actually pretty decent.
