In my opinion, I believe the economic model for desktop computing changed with the rise of mobile and cloud computing, and with this change in economics came a shift in focus from desktop computer platforms to these new platforms.

Before the rise of mobile and cloud computing, Apple and Microsoft heavily invested in macOS and Windows, respectively. I believe that Snow Leopard was the pinnacle of Mac OS X, and Windows 7 (which was released the same year) was also one of the best versions of Windows. The Mac was supported through hardware sales that subsidized macOS development, and Windows was supported through the sale of licenses largely to hardware manufacturers and businesses. These platforms were supported by developers, who paid for developer tools (I don't know much about the history of Apple developer tools before XCode, but I am familiar with the history of Microsoft Visual Studio, which has been the premier developer toolkit for the development of Windows desktop applications).

However, once iOS and its derivatives took off, and once Microsoft started focusing less on Windows and more on its cloud offerings, it seems like these new platforms become much more lucrative than macOS and Windows were. Thus, in my opinion, macOS and Windows have stagnated as Apple and Microsoft became more focused on their new money-making platforms: iOS and the cloud, respectively. Microsoft itself famously said that Windows 10 is the last version of Windows as we know it (https://www.theverge.com/2015/5/7/8568473/windows-10-last-ve...). Yes, there have been new versions of macOS and Windows since Snow Leopard and Windows 7, respectively, and there have been new features (most notably Windows 10's Windows Subsystem for Linux, which has attracted many former Mac users who bought Macs based on its Unix foundation), but later versions of macOS and Windows haven't sparked joy like Snow Leopard and Windows 7 did.

What also isn't helping matters, especially for the Mac, is the rising popularity of cross-platform applications that do not conform to the UI guidelines of their host platforms. While the Windows ecosystem hasn't promoted UI consistency for a long time, one of the strongest traditional selling points for the Mac was UI consistency across applications and most applications' conformance to the Apple Human Interface Guidelines. But when so many new desktop applications are written in Electron these days, how can the host operating system tout its unique advantages over its competition? I understand the rationale for cross-platform applications; (1) it is cheaper to develop and maintain a cross-platform app than to develop and maintain native ports, and (2) the vast majority of computer users are not concerned about adherence to platform UI guidelines provided that the software is still usable. In a world where so much computing is done in a browser and when an increasing number of native apps are written in cross-platform frameworks, the choice between Windows, macOS, Linux, and other operating systems increasingly comes down to which operating system best supports the hardware its running on. I think Apple knows this, which is one of the motivators for Catalyst (https://developer.apple.com/mac-catalyst/), which facilitates the porting of iOS apps to macOS, and a new feature that allows iPad apps to run unmodified on M1 Macs: this helps differentiate the Mac from competing operating systems.

I expect to see more of this in the future. Unfortunately I think the days of a polished desktop environment like the classic Macintosh or the Mac OS X era, where there's a major emphasis on platform uniformity per adherence to UI guidelines, are numbered, simply because today's economic incentives promote either walled gardens on one side or the Web and cross-platform desktop applications on the other side. I commend Apple for trying, though, before it discovered how lucrative iOS is.