The first semiautos were based on existing designs. Browning's first semiautomatic was a modified lever action rifle that used gas to work the action, and that required smokeless powder. A gas operated BP weapon would require complete cleaning after very few shots.
The idea of a simple blowback action may seem obvious in retrospect, but it really required very original thinking. Who would have thought to design a rifle in which the bolt was held in battery by a spring?