So the best hope may be philanthropists like Bill Gates...
No. I don't believe that private charity alone, however much wealth it might amass, can ever deal with such a vast task. IMO a more realistic best hope would be the benevolent, well-controlled development of and a fair distribution mechanism for all kinds of intelligence-enhancing technologies, from currently existing medicine up to genetic manipulation and more radical transhumanist stuff like cranial augs. Today there would be a heated political debate as to whether those would be best achieved by market- or government-dominated means, but I'm afraid that both would be grossly inadequate for the humanitarian purpose of prioritizing the most disadvantaged first. People of <85 IQ don't tend to have large incomes or political pull and strong communities, so without someone having the will to "uplift" them on a global scale little could probably be acheived. Which, as with the other infamous Gordian knots of today, should make us consider whether trying to ensure a positive Singularity might not be the most effective and ethical course.
The remarkable observation that medical spending has zero net marginal effect is shocking, but not completely unprecedented.
According to Spiegel in "Too Much of a Good Thing: Choking on Aid Money in Africa", the Washington Center for Global Development calculated that it would require $3,521 of marginal development aid invested, per person, in order to increase per capita yearly income by $3.65 (one penny per day).
The Kenyan economist James Shikwati is even more pessimistic in "For God's Sake, Please Stop the Aid!": The net effect of Western aid to Africa is actively destructive (even when it isn't stolen to prop up corrupt regimes), a chaotic flux of money and goods that destroys local industry.
What does aid to Africa have in common with healthcare spending? Besides, of course, that it's heartbreaking to just say no -