A while back, Tim Cook came forth and set the record straight; Apple is not interested merging macOS and iOS. There was a time that rumor was so rife, and we all believed it makes sense since Apple’s biggest competitor, Microsoft went that route with Windows 10 on desktop and tablet.
However, that does not mean you will not get to enjoy your favorite iOS app on your Mac. At WWDC 2018, Apple released a new developer tool UIKit that blurs the line between iOS apps and macOS ecosystem. The tool will allow you to bring iOS apps right into your Mac; for work or pleasure. Though the tool will not bring all iOS apps to macOS, but a decent number of them. Probably the most popular ones at first.
It seems Apple is adamant that there will be no merging of macOS and iOS, yet they do recognize (to some extent) the need for cross-platform interoperability. With UIKit, they have sort-of given into what those wishing for merger, albeit halfway.
There is already some form of merging of the macOS and the iOS; especially when you look at the upcoming macOS 10.14 Mojave update. The macOS Mojave comes with iOS apps like Apple News, Voice Memos, Home, and Stocks. These apps also come to macOS with the same look and feel they have on iOS. Though they are better optimized for viewing on bigger screen and navigation using mice and keyboard.
The UIKit will be availed to developers starting next year, and that is when you expect to see much of these macOS – iOS apps crossovers.