I think this is the strongest argument, but the sticking point is still the interface actually integrating with the OS environment well. Otherwise Java/C# would have taken off as a de-facto portable solution decades ago (and, in fact, this seems to work pretty well for stuff like games where native integration isn't a factor—most of my favorite games these days run on some sort of managed code, and in fact modding would be much more difficult without this).

Granted, at least the browser code uses native widgets, but the look/feel and layout still tend to be way off.