Agreed. Just look at Microsoft after Gates. Or Intel after Grove. In both cases, they turned things over to operational, #2 types in Balmer and Barrett and in both cases, the companies stagnated.
And in both cases, the transition was right before the industry-wide dot-com bust, and in both cases, the company is still huge and profitable and nearly a monopoly. So Apple's probably not doomed. They just might stop being the source of the next big thing. Apple's exponential growth has to end sometime, and somebody's going to be unlucky enough to be CEO at that time.
The difference is while apple has made a ton of money, they don't have the dominant control of a market the way a company like MS did when Gates stepped down. Monetarily sure they're stupid successful but it's still quite easy to leave most of their platforms, the biggest exception probably being iTunes due to people being used to using it to manage their music, no matter how shitty the app is.
Apple aren't even trying to make it hard to leave their platforms, they're trying to make you not want to leave them.
Well, they're definitely failing with iTunes. It's the one Apple program I use and, bluntly, it's rubbish.