[WalterBright]

Wealthy people do not care about tax simplification, they care about how much in tax they have to pay. So the trade was lower tax rates overall in exchange for giving up the low tax rates that made tax shelters extremely attractive.

Your position underestimates how pervasive tax avoidance using shelters was at the time. The wealthy were never going to give that up for simplification, after all, they could have simplified their taxes anyway by not investing in shelters if that was their issue.