If it leads to decades more time of fertile reproduction and productivity, it needn't do so. We'll still have some rapid diseases not related to age, and of course accidents. If some of our brightest minds have another 40 years at the peak of their scientific and engineering careers, maybe we'll have seasteading and urban hydronic/aquaponic/aeroponic food supplies figured out and affordable. Convincing people not to have another generation at 20 years of age would of course be the hard part.