This is already the case. All of Apple's operating systems run Darwin under the hood, and things like iOS and watchOS and tvOS have been this way since day one (macOS has been this way since OS X but prior to that it was a different situation).
en.wikipedia.org
And they've been released on schedule with each other for years now.
And the concept of Universal Purchases basically means all the App Stores are the same now - you can buy an app and get the iOS, tvOS, watchOS and macOS versions all at the same time.
So... done and done. This is why a lot of people aren't even going to notice this change.