But...
Surely all those who knew/know enough to download and install an alternative browser would have already done so which leaves those lacking that knowledge making up the vast majority of any switchers.
So, an MS update pops up asking if they want to switch to a different browser. Many probably just clicked on the first in the list without a clue. When said browser was then installed do they then get a similar popup asking if they want to switch back to the other browser or are they left then with whatver they chose? The later I bet. Probably many don't even notice the difference...
Me? I chose the ignore the update and carried on using IE8. Cue the usual barage of M$ and IE bashers as above telling me how crap it is except, if they actually knew anything at all, then they'd know that in protected mode IE8 is safer than their precious FF...