Even if iOS devices completely eclipse the Mac lineup, it can stay around as a profitable business for quite a while:

All the big new features in Mac OS X recently have been driven by or also developed for iOS (Core Animation, OpenCL, GCD, Exchange support, etc.), so clearly it won't cost Apple much to maintain Mac OS X. The desktops and laptops have been a fairly slow-moving target lately, and particularly for the laptops they haven't been doing much in the way of revolutionary hardware design since introducing the unibody enclosures. Apple's relatively high profit margins also mean that sales can fall to a very low level before a product line becomes unprofitable. When you also consider how much easier it is to maintain Mac OS X as an iOS development platform than to make Windows a good development platform (due to the strong similarities between iOS and Mac OS X and Apple's control over both), and when you consider that Apple doesn't have any trouble attracting iOS developers in spite of the Mac-only development tools, it's pretty clear that the Mac can keep making Apple money, even if they don't put much work in to it.