the beginning of a decline in Windows dominance
Windows dominance is going nowhere fast Octane.
Microsoft may well be reaching a plateau in terms of Windows platform adoption, but the stability and size of that plateau should not be underestimated. It's going to be a very long time indeed before you see the decline of Windows.
(And that statement comes from someone who would rather sit in front of a Mac than a PC any day, so I'm no Microsoft spokesperson !).