the RoI is still terrible compared to comparable interventions like hiring a personal trainer, diet coach, personal chef, etc. that could have forestalled the need for a heart transplant.

Even having purchased all those, a person may need a heart transplant. Genes, disease, accidents, and nurture while young (and unable to choose one's own lifestyle) all strongly influence the eventual need for a heart transplant. So for many people, even a lot of lifetime investment into their health won't mean the RoI on a heart transplant will be bad.

Also, at the point where you choose whether to have a heart transplant, the RoI needs to be compared with other things you can do with that money during the time you have left to live without a transplant. If you have a lot of money, and the transplant improves your QALY, then the RoI is likely good.