From MacRumors.com - June 2004 Live WWDC Expo Coverage:

Now, onto Mac OS X. 12 million people on OS X. About 1/2 of our installed base. 12 thousand native apps.

So we've got 24 million Mac users and 12 million are still using OS 9 -- more than that! That's just "installed" user base. I've got OS X on my iBook 500 but don't use it much anymore. Everything in OS 9 land is so much faster (okay, yes, I use OS X daily on my iMac, but the point's the same; OS X has issues on low-end machine, enough that many likely use "super-speedy" OS 9). So even some lower-end Macs that shipped with OS X probably still have users booting into OS 9. Not exactly a super adoption, but I suppose only in Apple-land could you expect more people to upgrade. Heck, Win98 is still pretty popular...