Sooner than you think.
Sheese, you're argumentative aren't you? I'll tell you now though, despite your fervent vocal objections that this very thing will happen. All other compiled hardware will become merely legacy, as legacy as logging on to a Renegade hosted BBS via your 300 baud modem firing up the ANSI colored text power of LORD, TRADEWARS, and the many, many flavors of MUD you still have active accounts on. Just as they were indispensable utilities that made the very corporate world turn, provided the light for it to see, filled it's lungs with their life giving rich air. Shortly following suit will be our current incarnation of super crutches like "Word", "Photoshop", "Quark", "Dreamweaver".
Oh you thought you had me there, any given operating system is software right? How could the browser even run anyhow without an operating system? They WILL be the operating system. You'll hardly even have to set up anything, because you won't have a hard drive, not a local one anyhow, why would you when you can have 100 backups of all of your data everyday from multiple places in the world encrypted for your privacy? Oh graphics cards you say can't just expect everyone to be happy with the same one. That too will be obsolete and no I'm not even pushing that a break through in quantum computing will make for a limitless cap, even with technologies today we could create such a processing beast, the same, (although more refined), way Google is able to search through 6 billion indexed sites in a mere moment our web hosts will be able to serve graphics to their graphic design clients or simply their game players.
This is NOT a pipe dream... Oh wait I lied, that is exactly what this internet is and that's exactly what is needed to make this all start to fall into place, a slightly wider pipe. Were on the brink of not only having all human knowledge available but all human resources able to pool together on projects. Can you imagine the possibilities?
I know I know, calm down there buckaroo your telling me, its probably a lot further off than I think. Hey could be, I'm to excitable really, who would go for a completely online OS anyhow?