But that's already the case. There are people right now who will live longer than others because they have access to better healthcare.[1] Why should it be strictly better (addressing only your point b) for the situation to change from "people with an average lifespan plus people who live 10 years longer" to "people with the same average lifespan plus people who live 20 years longer"?
Or in other words, is longevity some sort of unalloyed good? Is arithemtic the only tool needed to judge the merits of life extension? Another comment asked us to consider what would happen if Einstein were still alive; maybe there are some other people whose continued existence would not have been a net benefit to humanity?
[1]Yes, as mentioned elsewhere in these comments, people on welfare have access to some healthcare, but after going from a premium employer-paid insurance plan to what's available under my state's Medicaid expansion, I can guarantee that the former has a higher standard of care.