The 10% stacks additively, each time you do the deal, this status is applied to a different 10%. If you sacrifice 10 fingers, the entire world is guaranteed basic human needs (food, water, shelter, clothing, healthcate, etc…)
Nobody knows you saved them. Would you do it?
If everyone knows about the fact that you can do this deal, would that change your mind?
This is what I think when characters in fiction want achieve inmortality. Yeah dude, inmortality is one thing, but good health is other thing. You can get disabled, unable to walk, even worse, been in a vegetal state, but hey, you are inmortal.