One possible outcome is that you donate more before getting the same net return. It really depends on the target metric: Do you want to donate at least x? Do y

One possible outcome is that you donate more before getting the same net return. It really depends on the target metric: Do you want to donate at least x? Do you want to net at least y? Or do you want to pay at most z in taxes? Only z would cause reduced contributions from shrinking tax rate. Unless of course there is some disguised return channel hidden in the charity and those contributions are not as selfless as they are supposed to be to legitimate deductibility.