I think you're missing a few key details. Populism alone is not enough to bring about authoritarianism, otherwise the number of democratic countries would be decreasing, not increasing, over time. In many of the populist revolutions you mentioned, the most important catalyst was a broken economy that had put the majority of that country's population in poverty. Hitler may have been a great orator, but if post WWI Germany was economically sound and not the whipping boy of Europe, many would have dismissed him as the nationalist extremist that he was. What really drives populists revolutions is watching your friends and neighbors dying of hunger or exposure because they cannot afford to take care of themselves.