Didn't Sun try (multiple times) to sell useless bricks... err, "network computers"? Here's Google, trying the same thing for gosh knows what reason. This example of the baseline problem is pretty good:
But, there are some things you can't do strictly on the Web--like play Portal 2 or Minecraft, and living completely in the cloud takes a significant culture shift. There is also a huge trust factor with storing your entire life online, and the small issue of what happens when you can't get connected to the Internet and your Chromebook is a glorified paperweight.
Business travelers often need to work on planes, for instance - and it's a minority of flights out there with a network connection (and when they do exist, they are sloooooooow).