iOS and Android don't look like Windows killers today, and they aren't. But they are harbingers of things to come. In 1975 the MITS Altair didn't look like a mainframe / mini-computer killer, and it wasn't. In a mere 10 years since the Altair (a toy computer) hit the shelves the entire industry had been overturned, and mainframes/mini-computers were dying out while micro-computers came to the fore. Micro-computers grew up a lot in that time frame, but the essential elements of their superiority were there from the beginning (simpler, cheaper, more mass-production friendly, performance tied to components not overall construction, faster/cheaper innovation loop, etc.)
The same is true for mobile OSes and for web-apps. They have a lot of growing up still to do to compete with Windows everywhere, but the core is still there, and they have far less to grow than the Altair did. In another 10 years how much more sophisticated will be the iPad, iOS, android tablets and phones, and their native apps etc? In another 10 years how much more sophisticated will be webapps? In 10 years a VPS (or equivalent "cloud" instance) with hundreds of gigs of RAM and a terabyte of SSD storage and the CPU power to match will probably cost the equivalent of $20 a month in today's dollars. Add to that powerful new technologies like web sockets and all the other html5 technologies, along with new things we haven't even imagined yet. In 10 years gmail will be nearly 3x as old as it is today, imagine the innovations that will happen in that time.
And I'm supposed to imagine that somehow there's no chance that Windows will lose its footing amidst all this innovation? Perhaps it won't, but I can't predict the future, and from what I see it looks like there are huge very real risks to Microsoft's cash cow.