To fit the retrofit loom, you only need to remove seats and a few panels and lift the carpet a bit.

To fit the original loom, probably all seats, remove carpet or lift a lot more, and also remove a lot more of the dash. Maybe all of the dash. And possibly unravel existing loom. I've never done it so I'm not sure exactly.

Remember, the original factory loom - which is for nav and many other things - is fitted to an empty metal shell of a car, and then everything added on top of it.

The retrofit loom is designed to be added to the car as an optional extra after the car is assembled, and is thus much much much easier to fit.

BMW issue retrofit instructions for their retrofit kits.
The do not publish instructions on how to replace factory loom other than their normal vehicle service instructions.

Refer to the WDS for loom layout and you'll see how the original loom is wired. Note location of connectors - and you'll get the idea as to where each wire goes, and what a job it would be to fit the factory loom. Personally - I wouldn't do it!