Not even tablets will run on top of browsers.
Developers will never give up a real operating system. You just can't develop on consumer devices like the iPad; these are designed for consumers, they come with many limitations to make the consumer's experience easy and smooth, but at the cost of hackability.
I always hesitate to use the word never. Things can change very rapidly. What developers need now may not be anything like what we need in 5 years.
But more importantly, what a "real operating system" is will probably change in the next 5 years.
Look at photographers. Despite all the consumer level digital cameras that have flooded the market, professional photographers (and even hobbyists) still buy the more expensive (and more complicated) cameras that a consumer would probably never buy.
Some people use to cling to assembly instead of using C. Now days it's rare to find someone who will use Assembly over C.
Your argument would work if, say, people start writing operating systems in Python.
Another important thing to realize: there's a big difference between the OS and the GUI. The GUI might very well be implemented on top of "web" technologies, but that's not enough to say that the OS is the browser.
These aren't all GUI related things. I strongly believe you can now have a OS that is basically booting you into a browser and all OS interaction would go through WebAPI and still build most applications we use today.Obviously there will always be cases which you need to drop down a level.
Developers of ChromeOS or boot to gecko seem to agree somewhat.
Have you seen Mozilla's 'Boot to Gecko' project?