Well, if Microsoft delivered more value to shareholders (and to its employees), then those folks could choose to contribute that money to a variety of sources,

Well, if Microsoft delivered more value to shareholders (and to its employees), then those folks could choose to contribute that money to a variety of sources, philanthropic and otherwise. I'd personally think that the Foundation has done more good than a better MSFT would have, but suppose (1) billg delayed retiring by 10 years (2) we assume that billg's presence would have drastically improved MSFT's fortunates, then I think it's plausible that it would be a net benefit for society.