I definitely don't see what Gates and Buffet are doing as "legacy building" - if they wanted to build a "legacy", they would create some self sustaining "family

I definitely don't see what Gates and Buffet are doing as "legacy building" - if they wanted to build a "legacy", they would create some self sustaining "family foundation" that acted more like a social club than a charity. Instead, they are explicitly stating they want to give most of their money away while they are alive and all of it shortly thereafter.