When we started the HTML5 exporter no browsers hardware-accelerated it so it was really slow everywhere. Now, with WebGL, it absolutely flies, it's just mobile is a little behind desktop.
In page 2 of this very discussion.
And what Ashley means by "when we started" is about a year ago (give or take a few months).
So your "5-10 years" range is quite pessimistic imo.
Give it a couple of years to have became more accepted as "selling-hits" games will have appeared.
Also in 5-10 years you intend to still have your actual old gear as main computer ?
Let me hope that by then you'll hopefully have upgraded for better hardware, with better OS all designed in a world where HTML5 is a reality.
In the end it all falls back to this idea that "games in browsers aren't true games", or that "casual" games are inferior to "exe games".
Let's hope that most of the entries to the rotary comp will tend to prove you wrong.