alt Hacker News> I'd say the decline came around 2012-2013 or so.
I think it started slightly earlier: 10.7 Lion in 2011 introduced the new full-screen mode that was completely broken on multi-monitor setups, as though Apple entirely failed to test on or even anticipate what was at most a moderately "power user" hardware configuration. They've introduced lots of useless features over the years (eg. Game Center), but that full-screen mode was the first time I recall OS X having such an in-your-face usability regression that was so obvious and avoidable.
10.7 also dropped Front Row, which was a disappointment to me, but is at least understandable in the context of Apple TV existing as a separate product they wanted to steer users toward. Losing Rosetta in 10.7 was also somewhat justifiable, and didn't hurt me much since my first Mac was an Intel machine and I didn't have much of a library of PPC-only applications.
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The problem is, in the age of the Internet, old operating systems decay. Even MacOS 10.13 is effectively unusable as a primary workstation, NOT because Apple has abandoned it, but because Firefox, Chrome and Homebrew have abandoned it. Yes there are alternatives, but my point stands.
I'm a Linux guy who doesn't really like Macs but has intermittently been required to use them. On the whole I have a grudging respect for Apple (their hardware is peerless), but seeing one screen turn to "brushed steel" when the app on the other was put into full screen mode kind of blew my mind because "UI is worse than Windows" was not, at the time, a failure mode I believed the company was capable of.