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.
To me it's all contingent on whether your brain is healthy, functioning properly, ideally at an optimum. People who talk of Life Extension say that improving 'healthspan' and lifespan are connected.
Probably society needs a range of brain ages to function, or has done until now at least. That said there is no possible utility to people getting degenerative brain disease, I don't see the point of keeping the vehicle if the driver is dead.
Also consider the tasks a person does in life. A blue collar man dies much younger than rich woman, I think 20-30+ years. Some of that may be avoidable through better healthcare or technology or simply unavoidable because of genetics but it's also hard to imagine intense physical labour doesn't affect your metabolism in such a way that it ends your life sooner.
As a blue collar worker I'm becoming aware of this drawback..!
On the other side I know workers who've worked physically demanding jobs who are in their 90s and are still trotting about, have full head of dark hair.