What would the point of "adding functionality to the OS" be? If they wanted to put macOS on iPads, they'd just put macOS on iPads.
My understanding at Apple's strategy here is that they're simultaneously exploring two different GUI paradigms — almost pitting them against one-another to see which wins (or, if you like, making a hedged bet):
• macOS, for building "Unix pipeline-like" workflows where you point different programs-as-tools at the same document or take one program's output as another's input. Apple encourages macOS developers to make this kind of app.
• iPadOS, for building "all-in-one silo" workflows (think: Photoshop, Garage Band, XCode), where the developer intends to solve fully for a use-case, such that people with that use-case can get by using only their app. In these cases, rather than interacting with other apps, a siloed app will embed whatever other accessory workflows a user might need, either directly (e.g. XCode embedding a terminal console) or through plugins (Photoshop plugins, Garage Band VST support.) The user might use other apps at the same time as this app, but not in a way where the apps are sharing data or interacting in any way; rather merely using each app to "do what it does" — e.g. referencing a design diagram in Miro while implementing that design in XCode, and writing down reminders in some reminders app. (Thus, the iPadOS 16's Stage Manager, which assumes you want several apps on screen at once, but doesn't implement drag-and-drop between apps or any other kind of useful inter-app interaction.)
As a user, as long as each user-story you have has been perfectly addressed by some particular siloed iPadOS app, then iPadOS should work for you. (And there are a lot of people whose user-stories have all been perfectly addressed by these siloed iPadOS apps — mostly, people with boring, predictable, traditional workflows. Novelists; illustrators; business executives; possibly salespeople.)
However, if your workflow is niche or "constantly reinventing itself" enough that nobody's ever going to make a siloed app specifically for your needs, and so you expect to get things done by throwing files between various different tools all day — then iPadOS is never going to work for you. You need a desktop OS designed around that kind of thing.
