The problem i see with Trotzky leading the USSR isn't that he would have underestimated fascism, it's that he was really awful at getting the party on his side. I mean, sure, in a scenario where he was in charge during WW2, that obviously would not apply for some reason, but that brings us to the core of the problem with such scenarios: A Trotzky leading the Soviet Union wouldn't be the historical Trotzky, it would be some other dude looking just like Trotzky.