Horace Dediu, "Wherefore Art Thou, Macintosh?":
The key to the Mac therefore becomes that which the iPad/iPhone isn’t: an indirect input device. The keyboard and mouse/trackpad are what define the Mac. The operating system, the apps, the UX, are all oriented around the indirect input method. The iPhone’s capacitive touch brought about the direct input method, a third pivot in input methods (first was mouse, second trackpad/scroll wheel). Each pivot launched a new set of platforms and the Mac is the legacy of the second.
It’s not obsolete but it is a decreasing share of engagement. Alternate ways of doing the jobs it does well with direct input are emerging on the third pivot but they are not yet good enough. The children are still adolescent and making lots of stupid mistakes. There’s still life in the parents.
The parent-child metaphor is so apt. iOS is the future, but the Mac remains important.