Eventually the market may force their hand, much as they did with larger screen phones. Pretty much all Windows notebooks above the bargain basement range have touch screens, whether users need them or not. That said, it's possible that Apple will build up iOS' capabilities rather than add touch to OS X. For example, they could add side-by-side multitasking, a file system, and better USB connectivity for peripherals (likely through an adapter or dock). Right now, iPads have functional versions of iWork, Office, and Adobe products. A lot more people use iOS than OS X, so that may be the easier move.