I think it's good that taxes can solve problems a decentralized, and therefore disorganized, system will struggle with, but in the same way i don't think that a centralized economy will run as efficiently as a free market, I think philanthropy will make greater gains if a variety of parties are allowed to spend their money on what they think is important without needing collective approval.
You’re discounting scale though. Just like Microsoft destroyed competition and innovation with their monopoly (only possible with government backed IP laws), Gates eliminates competition for philanthropy and policy by force of his fortune. Gates is centralizing things, around himself.