Life Extension versus Replacement

Has any­one here ever ad­dressed the ques­tion of why we should prefer

(1) Life Ex­ten­sion: Ex­tend the life of an ex­ist­ing per­son 100 years
(2) Re­place­ment: Create a new per­son who will live for 100 years?

I’ve seen some dis­cus­sion of how the util­ity of po­ten­tial peo­ple fits into a util­i­tar­ian calcu­lus. Eliezer has raised the Repug­nant Con­clu­sion, in which 1,000,000 peo­ple who each have 1 util is prefer­able to 1,000 peo­ple who each have 100 utils. He re­jected it, he said, be­cause he’s an av­er­age util­i­tar­ian.

Fine. But in my thought ex­per­i­ment, av­er­age util­ity re­mains un­changed. So an av­er­age util­i­tar­ian should be in­differ­ent be­tween Life Ex­ten­sion and Re­place­ment, right? Or is the harm done by de­priv­ing an ex­ist­ing per­son of life greater in mag­ni­tude than the benefit of cre­at­ing a new life of equiv­a­lent util­ity? If so, why?

Or is the tran­shu­man­ist in­differ­ent be­tween Life Ex­ten­sion and Re­place­ment, but feels that his efforts to­wards rad­i­cal life ex­ten­sion have a much greater ex­pected value than try­ing to in­crease the birth rate?

(EDITED to make the thought ex­per­i­ment cleaner. Origi­nally the op­tions were: (1) Life Ex­ten­sion: Ex­tend the life of an ex­ist­ing per­son for 800 years, and (2) Re­place­ment: Create 10 new peo­ple who will each live for 80 years. But that ver­sion didn’t main­tain equal av­er­age util­ity.)

*Op­tional ad­den­dum: Gustaf Ar­rhe­nius is a philoso­pher who has writ­ten a lot about this sub­ject; I found him via this com­ment by util­i­ty­mon­ster. Here’s his 2008 pa­per, “Life Ex­ten­sion ver­sus Re­place­ment,” which ex­plores an amend­ment to util­i­tar­i­anism that would al­low us to pre­fer Life Ex­ten­sion. Essen­tially, we be­gin by com­par­ing po­ten­tial out­comes ac­cord­ing to over­all util­ity, as usual, but we then pe­nal­ize out­comes if they make any ex­ist­ing peo­ple worse off.

So even though the over­all util­ity of Life Ex­ten­sion is the same as Re­place­ment, the lat­ter is worse, be­cause the ex­ist­ing per­son is worse off than he would have been in Life Ex­ten­sion. By con­trast, the po­ten­tial new per­son is not worse off in Life Ex­ten­sion, be­cause in that sce­nario he doesn’t ex­ist, and non-ex­is­tent peo­ple can’t be harmed. Ar­rhe­nius goes through a whole list of prob­lems with this moral the­ory, how­ever, and by the end of the pa­per we aren’t left with any­thing work­able that would pri­ori­tize Life Ex­ten­sion over Re­place­ment.