The original Mac released in 1984 and was produced through 2000, sixteen years. OSX released in 2000 and is now in its eighteenth year. Whilst each system has seen some evolution over time, the general metaphores and interfaces have remained consistent.
Apple have realised and internalised a core concept of GUIs: change is bad. There is a far higher cost to changing interfaces than can be gained through efficiency, and the retraining and unlearning costs are exceedingly high relative to benefits.
This is a message apparently lost on Microsoft and most of the leading Linux desktops.
Mind: I write this as someone who whilst using a Mac presently doesn't much care for the interface. My preferred desktop remains WindowMaker (itself based on Aqua's predecessor, NextStep), which has a key advantage of having changed almost not at all in the 20+ years that I've been using it. It's also configurable in ways I find useful, and I schlep around a configuration directory to new systems as needed.
That and a terminal window.