It already is. Check out OK GO's new gimmicky HTML5 website experiment slash music video and watch it bring your system to its knees.
Oh, only runs on Chrome too even though it "claims" to be HTML5 (meaning standards-compliant, meaning can run on Opera and Firefox -> I suspect these browsers can run it, except that the lazy devs hardcoded Chrome checks into it).
Check out Bjork's new funky HTML5 site too:
I pity the foo who tries to run that on a 600MHz machine.
HTML5 is actually slower and more bloated than Flash. The problem with Flash is not the runtime, it's the abusive content creators who don't know how to optimize content and leave you with massive preloaders, etc. (Looking at you, bloated Nike Sites!)
To see what I'm talking about how slow & bloated HTML5 is, check out Iain Lobb's Bunnymark benchmarking experiment and compare:
* the HTML5 version -> http://iainlobb.com/bunnies/bunnies.html
* with the Flash version -> http://blog.iainlobb.com/2010/11/display-list-vs-blitting-results.html
Almost everything you see being done in HTML5 today is something that's been done in Flash from 5-8 years ago. So, "the future" is actually pretty meh, retro. Oh, and am already hating HTML5/JS/CSS3 with the now unblockable obnoxious floating banner ads. At least with Flash, you can Flashblock em easily.