Interesting times.
Microsoft is a lot larger than is readily apparent. That's because in the enterprise they're about as entrenched as can be.
Take away excel and the impact would be about as bad as taking away google would be (or maybe even worse).
MS will be around in some form or another for a long time. The question is more whether or not they'll be able to leverage the remains of the old into something new. My guess is not as long as Ballmer is still around.
I work for one of the largest companies in the world and this is absolutely true. If Google went offline it would be a minor inconvenience. Taking away Excel would grind the company to a halt. I tried converting a few of our hefty xls files to Google Spreadsheets but its not yet robust enough for the task. And thats just the worksheets themselves - VBA and macros are another level entirely.
That said, I cant help but see trouble on the horizon for Microsoft's enterprise offerings. We recently transitioned to a MS hosted Sharepoint 2010 and its absolutely woeful. Office 365 is ok but not a killer app.
MS can afford to make some fuckups in this area but if they lose traction in the enterprise then I think its the beginning of the end. I guess the xbox division can keep them alive for a while.
But dont the big corps work on volume licensing that is renewed regularly? In any case there clearly isnt a rush of new customers.
That's the vast majority of offices. And the lower number that are large/savvy enough to be pickier on backend systems still buy Windows and Office licenses anyway.
On the flipside, it's hard not to see mobile platforms eventually cannibalizing Windows from below, the way PC platforms (like Windows and GNU/Linux) eventually cannibalized minicomputer platforms, which in turn had cannibalized mainframe platforms.