Well, in my environment I know a lot of people who switched back to something else. I also know some people who bought a MacBook but installed Windows on it and only use Window. (they only wanted the Apple logo
yup)
I could imagine that Apple sales would go down in the coming years, but usage?
And if you wonder why I said Apple sales: I believe with Yosemite and iOS 8 - and the ugly flat child look - a lot of people will simply not upgrade anymore. But these long time users will not switch to something else, I for one won't be upgrading anymore but I definitely won't start using Windows or whatever.
I also know some people who are looking for iPhones with pre-iOS 7 systems installed. I started to think about that too.