> You lowering your tax rate and giving that money to charity isn't magicking more money into the world, it is just a different allocation.
This is only ever true if you assume that government tax spending is 100% efficient, with nary a fraction of a cent being wasted. I don't think that's a safe assumption.
No. The assumption is that charity and government have roughly equivalent efficiency. Both government and charities have (wildly varying) overhead and government agencies may enjoy economies of scale that charities do not. Yet another area of the world that contains a surprising amount of detail.
This is only ever true if you assume that government tax spending is 100% efficient, with nary a fraction of a cent being wasted. I don't think that's a safe assumption.