This is a (common) fallacy that could be applied to any funding or focus decision. It reduces to "Why focus on X when Y is more important?". You could use your same examples here as an argument for why not to support space exploration, or environmental conservation, for example.
You only need social security if you’re unable to work. If we increase healthspan to the point that you can maintain the physical condition of a 30 year old indefinitely then you are able to pull your own weight.
Do you think you could work for 200 years straight? I suspect that'd be disastrous to mental health. Society will need to figure out PTO under the new regime as well, or additional research will need to be undertaken just so our minds can keep up with our foerever-young bodies.
Medical & aged care social security costs are tied to deaths of recipients: the bigger the delta between retirement and death,the greater the cost. If the average lifespan grows rapidly, current social security funding will become insolvent, requiring taxes to be raised and/or retirement age to bw raided to leep the current average delta: neither option will be popular with the electorate.
I think you misunderstand the intent behind longevity research and goals: It's to eliminate the senescence period, or to slow everything down if unable to achieve that.
