I'm not sure I buy your arguments, though. For people who earn enough to retire and live off investments, this could mean more time in retirement, rather than more time working. (Still good, but not a productivity boost.)
Also, I don't see a reason to assume that an anti-aging treatment would delay the end of fertility in women past 35 or so, or make egg freezing work for longer. Those would be separate medical advances.
Who cares? As long as they're not a drain on the system (compared to today), what's the problem? That sounds like a big plus actually: people getting to enjoy more time in their lives. Why would anyone not want that, unless they're some kind of religious nut who thinks this isn't "god's will" or something?
>Also, I don't see a reason to assume that an anti-aging treatment would delay the end of fertility in women past 35 or so, or make egg freezing work for longer. Those would be separate medical advances.
I don't think they'd be entirely separate. We can already freeze eggs, and women are able to carry children at older ages (sometimes not even their own children) thanks to IVF now. Anti-aging treatments should make this even better; women may routinely get their eggs frozen at 25, and then use them to make children at 75.
On top of that, now with people delaying parenthood so much, or not even having kids, we're facing demographic problems (too many old people being supported by too few young workers). If people have significantly longer adult lives, this could very well make it more feasible and desirable to have kids at older ages, which could stabilize the population problem. (People could have two kids at 40, and two more at 60, and two more at 80, for instance.)