The backbone of Apple's business is their merchant position in the AppStores and the iTunes media store. Not their hardware or software per se, it's about the system and control of the entire platform, while still enabling profits for developers and other third parties targeting their platform. Not at all different from how the way Microsoft operates its platform, except they didn't see the opportunity of bundling a package manager / store suite like how Apple does it, and they never had a chance to control the hardware platform except in the game console business.
Still, the core assets of a business is their data, hardware and software expenses are just the cost of the tools. They can do the switch to any other platform as long as they can somehow export and import their existing assets. If they fail to manage their assets, they learn a lesson unless they go under. It's purely a matter of tools, and the operating cost of the tools. Apple kinda beat Microsoft at their own game. It's not impossible for others to beat Apple at their own game, and at least Google is trying.
The main difference between an Apple user and Google user from their respective business perspectives are that the Apple user is still a customer, whereas the Google user is the product itself, whose usage patterns are sold to their customers; the advertizers. Microsoft is also still in the game, but I believe their strongest assets are bound to the x86 Windows platform, which I don't think has a very bright future. If they fail their ARM transition, they are gone. Outside the Apple/Google/Microsoft trio, there are very little choices from a typical corporation's standpoint. Facebook might still have a chance, if they play their cards right, but IMO the future of Facebook looks like the present of MySpace.