Several reasons why I don’t believe Apple will put macOS on iPad in the foreseeable future:
1. They’ve said so emphatically for many years now.
2. iPads—especially the new iPad Pros—have smaller thermal (and battery) capacity than MacBooks due to the thinner case with built-in heat-producing display, which is probably why they need to run iPadOS—macOS performance would suffer. M chip does help with certain iPadOS tasks, but for shorter periods due to hardware capacity.
3. Apple discontinued macOS on sub-13” displays because either they believe the UX isn’t good or not enough customers were buying it. And some people would revolt if Apple offered macOS only on the larger display iPad.
4. An iPad + Magic Keyboard is still two devices, so by having a dual OS that requires mouse and keyboard, you may not actually be saving much or any weight or hassle or money compared to a MacBook + iPad, depending on which models you get. In essence, you’re just subtracting the MacBook lid which is very thin and light—also useful.
5. I’ve never seen a good UX implementation of dual OS. Clunky at best. And dual boot goes against Apple’s philosophy of devices being always on and ready.
6. Probably not enough market demand for Dual OS, which doesn’t make it a good ROI.
The last one is probably the main reason, because if market demand was strong enough, Apple would set aside or work around all the other issues, like they did with larger phones and adding trackpad to iPad. It’s much more likely that Apple will simply continue adding functionality to iPadOS and making MacBook and iPad work better in tandem.