HTML5 is kindof awesome, but even the most awesome technology is limited by the number of people who can use it. Unless the W3C or Microsoft or Google or the Mozilla Foundation manage to convince the world to upgrade their browsers with the speed that Adobe can upgrade the install base of the Flash Player, HTML5 is always going to play second fiddle.
Now according to Adobe, Flash Player 10 is at 94% adoption in mature markets, and that's about... what, 10-12 months after release? The HTML 5 spec was formally named in January 2008, and the original started in 2004. Admittedly- corporate IT departments (the big evil) are as unlikely to upgrade the Flash Player as they are the browser, but if it takes that long for anything to make it into HTML, Adobe will have already had several upgrade cycles to react, improve, and move on.
Having said that: we can always return to the days of browser specific web sites, and that'd force people to upgrade: "This website is optimized for [Insert favorite browser here], please change your browser".