Now, of course, we tend to see fascism as right-wing, but I get the sense that that mostly has to do with the mainstream left's adoption of civil rights causes in the postwar era; at the time, it would have been seen (at least by its adherents) as a more syncretic position.

Indeed, many of the most prominent supporters of fascism came from the traditional left. Mussoloni was originally a socialist, Mosley defected from the Labour party, and they didn't call it "national socialism" for nothing. In fact part of the reason why communists and fascists had such mutual loathing (aside from actual ideology) was that they were competing for the same set of recruits. Then again, Quisling and Franco especially were firmly in the right-wing camp.

With such concordance from all sides of the political spectrum it's easy to see how one could conclude that totalitarianism was the next natural stage in history.