The "Classic" Mac OS UI developed incrementally over about 15 years, in a mostly evolutionary process. Even Platinum wasn't an especially dramatic change except in the context of that conservatism. But over the product's lifespan, it represented significant improvements.
Over about the same amount of time, OS X has ping-ponged around, undergoing a series of major reskins, without much in the way of consistent direction. It seems to be a lot of change-for-change's-sake, and it's not clear to me as a user throughout the entire period that the end result now, in 2016, is all that much better than where we started out back in 2000.
Apple's had nearly as much time with OS X's Aqua UI as they had to take the Classic interface from its start (512x342 1-bit pixels on the 128k) all the way to Platinum, and the results are not exactly impressive.
Overall Platinum lasted four years, which was just as long as the brushed metal phase between Panther and Leopard (2003-2007). Leopard was the next major revamp, which lasted until Yosemite (2007-2014).
So, if you wanted to be generous, you could divide the Mac UI eras like so:
1984-1991: Monochrome Classic
1991-1997: System 7 (colorized)
1997-2001: Platinum
2001-2003: Aqua
2003-2007: Aqua (Brushed Metal Remix)
2007-2014: Aqua 3.0
2014-pres: Flat Aqua
I remember installing Aaron when it first came out, and I don't think I ever uninstalled it (though I remember playing around with the Hi-Tech version that Apple's lawyers killed).