Hewitt's post hinges on that statement, presented without supporting evidence. But actually the Web is already dominant among the platforms Hewitt mentions (Web, Windows, iOS, and Android).
Relevant stats:
Web - 1.7 billion users as of July, given that 880 million people go to the single busiest website publicly measured by Google, and this site has a "reach" of 51 percent. (You can distill this same total from any of the smaller sites listed via simple division.) http://www.google.com/adplanner/static/top1000/
iOS - 38 million people as of April http://blogs.wsj.com/digits/2011/04/19/a-look-at-ipad-users-...
Android - 24 million people as of April http://blogs.wsj.com/digits/2011/04/19/a-look-at-ipad-users-...
Windows - 400 million Windows 7 license sold as of this month, the most popular version of Windows going. http://windowsteamblog.com/windows/b/bloggingwindows/archive... Even if piracy doubles total Windows 7 installs, you're still not halfway to total web users (which means trying to count creaky old XP as the same platform won't get you there, either).
Hewitt is a smart guy, ex Facebook, built their iOS client, so clearly he has a thought a lot about platforms. But he undermines his arguments about the future when he is incorrect about the present.