Once each person is no longer taken as an unquestionable given, eventually everyone will have to show that he/she has a life of net positive value (is justified in living) in order for it to be thought reasonable for him/her to go on living.
And there is no reason to think that his/her net value will be measured only against the (let us assume) zero value of death; it will be measured against the value that would be generated if the resources now being used to feed and clothe the person at issue were devoted instead to other, perhaps more useful, human beings (or animals, for that matter).
This is how we think today with regard to animal husbandry. It’s simple economic rationality. Once the “taboo” against killing humans is gone, it will be the iron logic of euthanasia for us, too.