Awesome. I tried finding some numbers myself, but was disappointed in the lack of data. Would be glad to see what you find...
If someone started at version 4 and they've kept current all the way through to version 7 then that would be the kind of stuff we're looking for.
After all, the point I was trying to make was that it is very hard for people to upgrade across major versions, so plenty of them are stuck with whatever was 'hot' when they first started out.
The knowledge required for migrating drupal to a higher level version is a lot more involved than for building the website in the first place.