Apple has denied merging macOS and iOS, but IMO a universal OS that works on all your devices is inevitable.

Mobile devices are now powerful enough that they could connect to a screen and display a desktop environment to perform everyday tasks. I'm almost certain Apple isn't doing this because it would canibalise its products.

If the rumoured Andromeda OS by Google is what I'm describing, macOS and Windows will become the trucks of desktop OSs.