With no burdensome income tax, you don't need interest mortgage deductions. And did you know that more than half of all charitable deductions are never claimed? So, right there, we have fifty percent that are unaffected. Ever study has shown that when Americans have more money, they give more. Bump that fifty up to sixty or so to take than into account. Plus you have to figure at least half of the rest would give even without the incentive of a deduction.
By my rough count we are cruising on at least 80% of charitable giving to continue as before... at least!
As for charitable giving, the high enders wouldn't donate as much. You want proof ? Once musem donations were tightened up / lowered , re deductability, they've all fallen off.