I believe there are larger effect sizes out there than metformin; metformin has a relatively small effect size on mice compared to other lifespan-modifying interventions, and the TAME trial chose metformin (as Barzilai admits) because it’s extremely safe and well-studied, not because it’s expected to be the best.
I agree with you; I don’t think aging research would be cost-effective at a trillion dollars of total funding. I expect that’s hugely more money than necessary.
It didn’t come from either, it came from me, as a benchmark for what seems conservatively possible to achieve in the near-term, and for the size of impact necessary to be plausibly cost-competitive with other causes like global poverty. (If the same amount of funding yielded only one year’s delay in the DALY burden of age-related diseases, I think that would make global poverty likely to be a “better buy.”)