How to manage birth rates may indeed become a difficult political issue with extended lifespans. But in purely economic terms, if you could get another 40-50 years of healthy productive career out of the same human instead of having another one spend 18-26 years growing and getting expensive education and then also needing a lot of extra healthcare in the later years, then that would be an excellent efficiency improvement. Also, if individual humans routinely expect to live >100 years, then that gives them a stronger incentive to avoid choices that have a long-term negative impact.