Sometimes, all it takes to be disruptive is to be cheaper.
"What market does the Chromebook create that a $200 netbook didn't already create?"
I prefer to think we mostly find markets rather than create them, generally speaking. Chromebook may find a market in India or Africa or China. Places where a mobile device was everybody's first experience with personal computing, so the idea of it being intimately tied to the network isn't so strange. And, just as importantly, where $50 is a weeks wages so it seems a lot more important to spend 25% less on the device.
I can see how me invoking Innovator's Dilemma here is possibly a stretch from some perspectives. Even if it is not disruptive innovation in the sense you prefer, it is still seeming more and more likely to continue to erode Microsoft's dominance in the places they have always been very, very strong: Cheap home PCs.
Also, I think the argument could be made that this is a better product for less money. Windows on such scanty hardware is a pretty rough experience. Chrome OS on tiny machines isn't so bad. And, for people who already only use their computer for Internet stuff, there are no benefits to the Windows experience and plenty of negatives.
So, this isn't a situation of, "Why would I get a dinky little fake computer if it's only $50 less than a good computer with Windows?" It's more of a "Why would I spend more for a computer that is slow, clunky, has a bunch of crap I don't need/want only so I can use also-ran webmail, online docs, etc. when I can get the market leading stuff for less?"
I dunno, really. I'm just trying to imagine this from the perspective of someone who didn't grow up in a world dominated by Microsoft, because that's the world we now live in. The world is now dominated by Google with Apple and Facebook as thought leaders on the periphery. Things are different now, and getting a computer with Windows on it just isn't a thing that (young) people care so much about.