Apple’s reputation for user-friendly software comes from the 1980s, when Windows was very primitive and when the Mac’s biggest competitor was MS-DOS, which was never known for user-friendliness. To be fair to Apple, Apple worked very hard to establish well-conceived UI guidelines and to ship representative software such as MacWrite and MacPaint to show how Mac software should behave.
In the 1990s Windows gradually improved, and Windows 95 was on par with Macintosh System 7.5 in terms of features and ease of use. It even had its own UI guidelines. Windows 95 was one of the factors that led to Apple’s troubles in the mid-1990s.
Even though it took over four years for the purchase of NeXT to lead to the first client release of Mac OS X in 2001, Apple distinguished itself from Windows PC vendors in other ways, such as ease of installation and Apple’s pursuit of the “digital hub” where the Mac was the center of a digital lifestyle involving music, digital cameras, and digital camcorders. This was the era of the iPod, iPhoto, iMovie, GarageBand, and related software.
Of course, Mac OS X solved the Mac’s long-standing stability issues, and Mac OS X also came of age when the Windows world was suffering with malware and security issues.
In my opinion, the Mac peaked in the mid-to-late 2000s, where Mac OS X provided users a solid operating system that was easy to use, and where Macs came bundled with a variety of apps from Apple that made it easy to do a lot of tasks many computer users care about, such as organizing music and photos, as well as editing music and videos.
Then came the iPhone and the tremendous profits that came from the iOS ecosystem, and with it came Apple’s shift in strategy, from the Mac being the digital hub to a hub focused increasingly on iOS and Apple’s cloud services. The Mac hasn’t been the main focus, and in my opinion the decline of Mac software is a reflection of Apple’s focus shift.
This is a really nice summary of the history of Mac OS (in both its incarnations).
While Mac OS X in the mid-late 2000s may have been a technologically superior operating system to Classic Mac OS, it was never as easy to use. The loss of the spatial Finder [1] along with the very strong UI consistency of Classic Mac OS apps (including 3rd party apps) left non-power-users behind forever. However, like everyone else in the operating system space, Apple didn't have to care about that because the browser took over and these users stopped doing things in native apps.
Classic Mac OS still can't be beat for working on projects in visual media. The persistence of the spatial Finder is so rock-solid that you can develop muscle memory for where icons will appear on the screen when you open a folder. This allows you to anticipate where they will be and move the mouse toward them before you can even see them (the zooming rectangles animation helps with this).
This method of working exemplifies the core philosophy of what the "desktop metaphor" was all about: having a spatial relationship with documents and tools on a physical desk lets you move your hands and eyes independently, grabbing and interacting with things without having to look at them. Apple worked extremely hard to bring this "illusion" to the Mac OS and no other operating system (including Mac OS X in all its versions up to the present day) has achieved this.
That's because the spatial illusion is very fragile and must be maintained with extreme care. Any loss of persistence (a window opening in a different place, an icon that moved or changed colour) shatters the illusion and puts the user into a defensive, "hunt and click" mode. Imagine cooking or working in a workshop and having someone re-arrange all your utensils or tools while you're away. Your entire workflow gets disrupted and your performance suffers.
Power users survived this disruption (in Mac OS X onwards) by moving to the keyboard and the Terminal, which have the rock-solid physical persistence of the keyboard itself to back them. Any time Apple tried to mess with the keyboard they got a ton of pushback from power users (see the touchbar on older MacBook Pros).
[1] https://archive.arstechnica.com/paedia/f/finder/finder-3.htm