More than anything, once people realize that they can be fine without MS because 5-10% of the non-Apple market has done so (and the alternatives has figured out the kinks with the mass influx of users) it could move from a trickle to an avalanche.
The upside is that MS has the reserves and fallbacks to get their shit together if they realized that they are faced with a bad sitation and those that can't leave will get better products.
I think they already know and have been trying not to lose to Apple for a long time, as evidenced by their awkward attempts to embrace “good design” and “cool”.
The upside is that MS has the reserves and fallbacks to get their shit together if they realized that they are faced with a bad sitation and those that can't leave will get better products.