I think the proponents of web apps only consider the twitter/facebook/etc type of apps in these arguments. If the types of game you're interested in are relatively simple affairs (Angry Birds/Cut the Rope/etc), sure, you could probably make it performant with HTML5. Good luck doing anything more complex (Unreal, PhotoSynth, GarageBand, etc) with javascript without at least 3-4 generations of hardware improvement.