Maybe 3-5 years until iOS/iPadOS and MacOS are essentially the same. It doesn’t make sense to keep them separate code based when iOS is developing into a hybrid form factor OS, with mouse, multitasking, windows, file system, external monitor, usb accessories, keyboard, etc.
My guess is non-pro laptops will basically be iOS, and then there will be a MacOS Pro, which unlocks some capabilities for developers, but all in a unified code base.
First step was to share processor architecture and work in the ability for iOS apps to run in MacOS. But iOS is very much converging into a replacement for MacOS. And just like iPhone iOS strips out features you unlock with iPadOS, same for the iOS-ification of MacOS.