The difference is that with a web browser, you can switch from IE to a superior browser and all your favorite web sites will still work (apps built for IE6 compatibility notwithstanding). This isn't true of switching from x86 Windows to some-other-OS on some-other-architecture -- your existing special-purpose business apps won't work. You have to switch everything out.

That's not to say that Windows can't get displaced, but that it's a lot different from displacing IE. You can't just switch from Windows and have nearly everything working the same from day 1.