Compared to the UI used between 1984 and the launch of Mac OS 8 in 1997, Platinum itself was a short-lived re-skinning. The 1984 UI was colorized in 1991 with System 7 and only fully reskinned in 1997, a 13 year span. Mac OS X is 15 years old now, for contrast--plenty of time to fit in a few reskins.
Not really sure the numbers make the point you're suggesting they do.
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.
Platinum was a very dramatic reskin at the time. It was originally designed to set Copland apart from System 7, but after Copland failed they just rolled it out in Mac OS 8. Aqua was then designed to set Mac OS X apart from Mac OS 8/9.
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:
Don't forget that Platinum was unveiled years before OS 8 was actually released, and one of the most popular System 7 extensions was Aaron, which provided the Platinum theme, and as such a large contingent of Mac users were using Platinum long before OS 8 came out.
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).
