I do believe that ultimately, MacOS and iOS will merge like you mention, although there may be some differences based on the particular device. They have already done some work in that direction with a certain level of app and feature support. I expect that is just the start of where things will end up. There is a hardware factor that's going to help Apple in this direction and that's the fact that they are moving toward all Apple silicon for processing and associated functions. If the computing hardware becomes all the same happy family, regardless of device type, the OS can also be unified.