Firstly, the Lion user interface makes extensive use of gestures. I purchased a Magic Trackpad a couple of months ago in anticipation of Lion. As far as I can see, MacBook users speak more highly of Lion than those using desktop hardware. I suspect that has a lot to do with using a trackpad (or not).
Secondly, Apple's track record with mice isn't exactly stellar. Opinions on the usability of the Magic Mouse seem divided (trying to combine two quite different functions in one device means compromise), the scroll ball on the Mighty Mouse/Apple Mouse is notoriously prone to clogging, and the 'puck' mouse supplied with early iMacs was widely derided.
I've been a fan of Microsoft's mice for many years. They aren't perfect, but I find them more comfortable and functional than Apple's designs. And I know a fair few very satisfied Logitech mouse owners.
Even if Apple did stop offering a mouse, that wouldn't prevent Mac owners who prefer a mouse for particular tasks or even everyday activities from using a third-party mouse.