I think if someone uses not-IE they've already demonstrated their willingness to go elsewhere if something better comes along.
I think that's an oversimplification, but how substantial or not I cannot say. From personal experience, I moved away from IE when I moved to Mac since Microsoft dropped IE for that platform. I suspect the number of people who've dropped IE because it's Windows-only is relatively minor, but then there are others that have been scared off by security threats. While no browser is immune, IE's ties to the OS make it more of a liability, and once people have rid themselves of that, they may just be happy where they are.
All speculation, though. I have no stats to back this up.