This is not true, but as a side thought to their ever expanding universe, I thought it would be an interesting concept and they would probably be the only ones in the world at the moment that could dint IE's dominance. How many of us have downloaded the toolbar? Begs the question does'nt it. ;)
Perhaps a lot of Alexa's installs are repeat customers. Due to a slowing of page loads, concerns about privacy, and the fact that Ad-aware complains about it a lot, I know I've installed and uninstalled Alexa at least 6-7 times, but I've hardly ever thought of uninstalling Google.
I agree that the advantages to the google toolbar probably make it one of the least unistalled toolbars out there. I have probably installed over 50 toolbars over my internet lifetime - and only google has stayed.
However, I don't really see the advantages of the toolbar making the trek to the browser itself.
Also - this really wouldn't be in google's field of interest as I see it either. But who knows...