I am curious abut this too. Apple obviously will never allow this to happen, but it is an interesting thought experiment.
The A12 is a decent processor, similar to the 2020 Intel MacBook Air, for example.
4GB RAM isn’t a lot, but macOS can run on 4GB even on Intel, so it should be doable.
64GB SSD is very limited, though for many users it could be enough for macOS and your apps, keeping the rest in iCloud.
You can add a monitor with HDMI and keyboard and mouse via Bluetooth.
If you look at it like this, while it would be a limited computer overall, it would still be a fantastic deal at just $199. Add a USB-C port and a 128GB option for $299 and it would be a fine computer for a lot of people, especially in school environments. Which is why Apple will never sell it of course (better sell Mac minis and iMacs with bigger profit margins).
Or do you think I miss something which would make this impossible from a technical point of view?