I certainly wouldn't go so far as to bet that Windows will go away, but I think that previous predictions about Windows demise were predicated on it being replaced on people's PC's with something else.

That never happened (and I agree may not ever happen), but I think the growing widespread adoption of non-pc computing devices (phones, tablets, tv's, etc.) definitely has a fighting chance. Within ten years, I'd bet that the vast number of people will be accessing the internet primarilly from something besides a PC.