[hythloday]

modern JS performance is perfectly adequate even for 3D games and the like

You can certainly write a 3d engine in JS, and it won't be laughably slow, but the idea that you're going to see any AAA games in the next 5 years written with any more JS than the usual Scaleform is extremely unlikely.

[_delirium]

I don't think there will be a wholesale replacement of AAA game engines with JS-based engines, but I do think JS-based engines will encroach more and more on AAA territory from the bottom up, as the visual-fidelity differences between AAA games and web/mobile/etc. games narrow. I know of at least one AAA-level game company that's working in-house on a WebGL-based game engine to hedge their bets, and currently debating what should be deployed on that platform. I would guess others are looking into it as well.