What Trotsky believed and why he believed it is almost irrelevant compared to the rhetorical position that someone assumes by calling themself a Trotskyist. The basic vibe of it was, "The USSR could've been good, but it went wrong at such-and-such point, so as it is now it's bad." If you erased Trotsky and all of his ideas from history, there would still be plenty of people adopting that position while calling themselves something else. It's a way of tempering support and gaining protection from criticism of a state's actions. We see a similar phenomenon with ultraleft Maoists opposed to the modern PRC. By holding up an improbable ideal of "What could've been," they malign the real material improvements delivered by the actually existing socialist projects, and frequently they'll be some of the first to criticize such projects in order to distance themselves from them. The perfect is allowed to be the enemy of good, and so they become de facto supporters of the status quo because nothing is ever good or pure enough to challenge it. Criticism of AES states is fine, but if you completely write them off then you're throwing away your only proof of concept that your ideology - which seeks to overturn the world order and bring war and instability in the short term - can actually succeed at making things better.