Don't forget that Platinum was unveiled years before OS 8 was actually released, and one of the most popular System 7 extensions was Aaron, which provided the Platinum theme, and as such a large contingent of Mac users were using Platinum long before OS 8 came out.

I remember installing Aaron when it first came out, and I don't think I ever uninstalled it (though I remember playing around with the Hi-Tech version that Apple's lawyers killed).