I'd say Mac is more likely. Not that Linux sucks, but you have to have some sort of Linux fanboy around to even know it exists on the consumer or small business level. And then that fanboy has to convince you to use it without annoying you to death (we all know that guy).
And Microsoft still has Enterprise in their back pocket. Enterprise gets much different licensing options than consumers or small businesses. And they have legacy software that just can't change. Things are moving more towards web based/platform independent software but that's a slow process. Heck, we are JUST migrating off of an AS/400 system for our ERP. Talk about resistance to change.
Consumers idolize Apple products and will move towards them. I'd see Small Businesses going towards either all Apple OR Windows/Mac/iOS/Android with Google products as the most likely scenarios. But even that change won't happen until after a few years of getting frustrated while sticking with Microsoft since it's all they know. Habits die hard. For my side consulting work, I recommend my clients go the Google route usually.
Linux needs an all in one package (hardware, software and mass marketing) to really succeed. Well technically, they already do: Android. It took the behemoth Google to even give Linux any mainstream consumer traction and still 95% of the people out there wouldn't even know it's Linux. Until this happens on the PC side, Linux isn't going anywhere further than it's already at. Which is sad because it's an excellent platform. Every year or so I have to try out the latest Ubuntu and wish I could realistically keep it.
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