Yeah, I think they're taking note of this.
Harder to develop because you can use the programming language and runtime of your choosing, including the exact same APIs that are on iOS, or harder because you don't have to cross-compile your app for an embedded platform and test it on a remote machine whose internals you aren't allowed to change or inspect? Maybe you meant harder because there's no convoluted chain of constantly-expiring certificates needed for testing and deployment, or because you have far more RAM, disk space, screen space, and CPU cycles available, or because you can run as root or even in kernel mode to extend the OS in ways that Apple hadn't imagined instead of playing along as their dancing promotional puppet app, which they can kick to the curb on a whim?
Or maybe you meant it's harder to support because users have to drag an icon (which they received from a web site in a single click) into their applications folder, instead of thumbing through 200 seemingly-identical app listings with terse descriptions and then fumbling to enter the same iTunes account password for the 90th time, re-agreeing to yet another slightly different revision of a 28-page-long terms of service and then going back and entering the same iTunes account password because iOS forgot you were actually downloading an app?
Ah, of course, now I see what you mean.
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.