Well, your body makes more plasma. It won’t regrow a kidney.
So let’s say that your remaining kidney craps out. (This isn’t rare, btw.) You are now on dialysis pending a kidney transplant, this time for you. In the meantime, ESRD (end-stage renal disease) makes you eligible for Medicare, so now taxpayers are supporting your sale-for-profit of your other kidney back in the day until we pay for your kidney transplant. Dialysis runs $70k+ per year. Kidney transplants run $400k. (These numbers are US healthcare funny money, but still, you are asking your fellow citizens to pay for this “easy money” choice that you made back in the day.)