Transplanted kidneys generally go lower in your belly than your "native" kidneys and they aren't swapped, the original kidneys are left in and they add another. Anyone who's had a transplant most likely has 3 or even 4 kidneys inside them.
They only remove them if they have some kind of immediate risk of damage to the body - Necrosis or similar. On top of that, while your original kidneys won't have enough function to sustain life, any function they do have will take some burden off of the transplant kidney(s).
It makes complete sense once I think about it. My mental framing is too often based on technology. Ie. there’s two kidney slots that you can populate with one kidney each.
If it helps, you can think of all medicine as reverse-engineering and hacking a machine the vendor has ceased supporting and won't provide documentation for.
