First: Your use of replaceability in regards to people is absurdly different than the standard meanings of the word eg: someone who is unexciting and less than capable. This is probably why you found it hard to relevantly Google.

Second: replaceability as a form of altruism has to compete with all other forms of altruism, as several commenters have mentioned. I put it pretty far down on the list of things someone should strive for if they're trying to do good in the world.