I seem to remember reading that Gates is still earning more than he is "giving away".
You could perhaps argue that his work in education will lead to a more capable tech workforce in the future, but even then, the returns won't go to him.
Fewer people in the world dying from polio and malaria? Not all returns have to be monetary, or measurable.
Edit: I'm just trying to understand why you think it's more precise to label what Bill Gates is doing as "philanthropic investment" as opposed to "giving money away."
For example, consider the following scenarios:
- I pay for my college.
- I pay for my son's college.
- I pay for my niece's college.
- I pay for a stranger's college.
- I pay the university directly so that more strangers can attend.
It doesn't seem like there's a hard line anywhere. Probably just two ways of looking at the same thing, which for me anyway has an effect on how I use my money (I don't want to "give it away" but am happy to "spend it" on making others' lives better).