What legitimately useful features can we expect out of the release of iOS five major versions down the line? Will there be any new features that change the status quo, and that we'd be practically unable to live without? The significant features like the app store and multitasking have already been done long ago. As for macOS, it seems that the new security controls and notifications are significantly impacting some people's user experience, but they're still "improvements" from Apple's standpoint.
And yet Apple and Microsoft are still somehow compelled to keep releasing new versions of their operating systems on a fairly regular basis. It's hard to imagine there will be a "final" or even long-term-support version of iOS. But those companies aren't going to market their new OS versions as changing nothing, so something has to change.
I'm honestly very curious to see what will happen ten years from now when the vendors will struggle to meet their own arbitrary standards for innovation with each new OS version when, by nature, there are not many more significant breakthroughs left to be had.