Apple doesn’t need to convice all the major media companies to offer their channels as a la carte options though an app-like setup. They simply need one channel (one piece of one company) to try it.
If Apple can convince one popular channel, say just ESPN (Disney) or Comedy Central (Viacom) or MTV (Viacom), to try a single subscription model through an in-app purchase, they’ll revolutionize the TV market. I think Apple is rightly convinced that a successful launch of one channel in an a la carte model would demonstrate the economic feasibility to everyone. Once successful, the test channel is certainly not going to back out of the popular and successful new business model, and others who want to grow their business will join in. That’s how you disrupt an old business model.
My money’s on ESPN because of Disney’s relationship with Apple, their status as the most popular channel on TV, the fact that they show a lot of live content, and that they’ve repeatedly proven themselves as an innovative and risk-taking media company.