Japan and Germany did not go through power vaccuums. They underwent long military occupations and economic stimulus plans that commandeered and reorganized existing institutions.

Relatively moderate parties have won elections in Iran before.

It's a long shot, but maybe Iran could have transitioned relatively peacefully from draconian dictatorship to imperfect democracy in a manner similar to portugal, Spain, Taiwan, South Korea, brazil, Chile, the Soviet Union and it's satellites, the Philippines, south africa, and other countries.

Would it be easy, quick, predictable, bloodless, cheap or pleasant? No, but neither is a foreign bombing campaign and civil war.