Critical Theory naturally predicts the outcome we historically observe of this bureaucracy becoming a new oppressive class

That’s not what a class is and Critical Theory as a historical ideological project always functioned to defang dissenting voices and produce “compatible leftists”. The Frankfurt school was funded by the CIA.

A Marxist-Leninist state voluntarily becoming stateless is as absurd as a capitalist state voluntarily becoming communist.

It’s not absurd, it’s just hard to imagine in the current historical moment where the strength of reactionary forces means, that it is far off, but necessary in the future.

Anarchism doesn’t oppress class interests, it unmakes classes so that the people who used to constituted them have interests that align with others. Within an anarchist commune, there are no capitalists to oppress others

No, but they are right outside the commune loading up their cannons to crush it like the Paris commune. If the defence is successful, it will have forced their will violently on the attackers, who constitute a different class (capitalists). Yes, only on defence, yes that is a legitimate form of organized violence. That’s the point.

And sure, people that volunteer a lot to defend others could become a class that can attempt oppression (whether as a junta, or just as demanding privileges for their noble task). But the same risk holds for any profession, and anarchy always works to subvert it.

Take that sentence and replace anarchy with Marxism. It doesn’t always work. There were historical failures and mistakes, as has happened in almost every anarchist project. For example the Spanish anarchists reversed their progress in woman’s liberation to appeal to liberals. It didn’t work. The kurdish anarchists sold out to the US empire and were betrayed by it again and again. Marxists did wrong too. But because of Marxisms principled stance based on material class analysis, it’s much less likely to dissolve, succumb to error or become a tool to reactionary forces.