Diminishing marginal utility is an axiom over fixed magnitude amounts of a good. Bill Gates still gets less out of an additional $10 even with new opportunities for philanthropy than a panhandler who is hours from starving to death and currently has $0. "Diminishing" doesn't mean it has to go to zero, but Bill Gates is not going to be in any situation where some number of dollars given to him on top of what he has means the difference between life and death.

Heck, this exact fact is the very reason why Bill Gates engages in philanthropy. He realizes that other people need the money more than he does.