It is not a "poor" architectural choice, it is a choice with advantages and disadvantages, and something that has been studied since the 1940s. See Harvard vs Von Neumann architecture. Harvard: separated, Von Neumann: combined.

Modern processors are usually a hybrid, Harvard on the inside (with separate instruction and data caches), Von Neumann on the outside. But some processors (ex: DSPs) are pure Harvard and both styles have co-existed since the very first computers, the ones made of vacuum tubes and relays. There is no "original sin".

No? Direct control is harder in a Harvard architecture, but often changing data is the attack you want to do anyway.

Many would say, myself included, that is not a poor choice. It was the better of two possibilities.