Ever since the iPhone came out in 2007 and almost instantaneously overshadowed the Mac, both in terms of sales and development resources, Apple has been making the Mac a bit more like the iPhone. Sure, a few features have moved the other way—the iPad has gradually gotten a bit more Mac-like as it has become powerful enough to do Mac-like things—but a big piece of every macOS release this decade has been “here’s all the stuff Apple brought over from iOS this year.”
Catalina moves macOS further and more decisively in the direction of iOS than ever; for the first time, third-party code written for iOS and iPadOS can run on the Mac with relatively few changes. At the same time, Apple remains adamant that the Mac and iOS/iPadOS are separate platforms that differ in ways that go beyond the underlying processor architecture or the primary input mechanism.
Catalina also draws clearer lines between the two platforms than we’ve gotten before. Apple has both said and done things that only make sense if the Mac will still be able to run whatever code you want for the foreseeable future, even as the default settings and security mechanisms become more locked-down and iOS-y. The overwhelming success of the iPhone indicates that most people are fine with Apple’s restrictions most of the time. But the slew of new desktop hardware we’ve gotten in the last couple of years suggests that Apple understands that a valuable, vocal chunk of the Mac user base (and the developers who drive the iPhone’s and iPad’s success) still wants powerful hardware that runs more flexible software.