Phones and tablets are very close to the point that desktop/laptops got to a few years back where unless there is some major disruptive technology introduced (eg. batteries with 100%+ efficiency over current ones) on top of them the one you bought this year is pretty much good enough to keep for the next many years, until it breaks somehow. There will still be a market of ultra-fashion-conscious buyers who really nee…
This is how it has gone with things like graphics, animation, even sound. In principle the core elements of the Mac OS don't require extremely fast chips/hardware ... but once you get into the fancy desktop animations, the sound processing, face recognition (e.g. iPhoto), etc etc, you require fancier hardware.
There are still lots of directions for Apple to push their software, directions that will be aided by better and/or more specialized hardware.
If all you need is a text editor, an email client and some CPU power for crunching spreadsheets then for sure, all you "need" is a desktop circa 2000 (or even earlier ...) ... but if you want to run the latest OS, even if you don't need/want all the features of that latest OS, you need to keep up with the hardware.
Never mind games.