Apple is already priced as a low-growth stock (P/E around 10). So if they never do anything more than leverage the brand loyalty of iPhone customers, they'll still fulfill the expectations of their price.

An adoption curve leveling off is a normal thing in business; money can still be made. When did the adoption curves for insurance, paint, homes, jewelry, soft drinks, etc. level off? Long before Berkshire Hathaway came along, but they have done pretty well in all of those of markets.

I think BH has a good chance of getting a return.