The reason they would consider selling a FUTURE interest in their organs is because it would be the best option on the table. Since it's not currently available as an option, they must be choosing considerably worse option now (which is the best currently available option).
How is your alternative more just, more fair, better?
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Imagine YOUR loved one received a liver/heart/kidney in this manner, thereby saving his/her life. And in so doing it DID improve the welfare of someone loved by the donor allowing him/her to get a better education, get better housing, or something else worthwhile.
What is so horrible about that? Your insurance company would cover it and it would make the world a BETTER PLACE for everyone. By the way, the surgeon gets paid a lot of money for a complex procedure like organ transplant; what's wrong with a donor getting a tiny fraction of that for his/her contribution providing the amazing, wonderful gift of life?