Wouldn’t a strong centralized state change the material conditions for the ruling party elite in control of that state?
we already live in a strong centralized state: capitalism. capitalism, specifically late stage capitalism, is the most centralized that class society will get, from there its only down hill. you are only analyzing these socialist states from a bourgeois perspective, unknowingly i assume. the proletariat and the bourgeoisie have an antagonistic relationship; in order to be democratic to the proletariat, you must be dictatorial to the bourgeoisie. form the bourgeois perspective, they can only see dictatorship and centralization, but this is the same case that the proletariat lives under right now.
As a result, wouldn’t they become, materialistically speaking, a new ruling class?
yes, they are a new ruling class, the proletarian ruling class, that's the point. the vanguard is the working class; in the USSR by the 1930s, the party was like 90-93% worker/peasant. lenin or mao were workers, they had a wage, they didn't own factories.
the vanguard is a natural development, every revolution, even an anarchist revolution, will have a vanguard. because there will always be uneven development of revolutionary understanding within the working class. not every worker will have the same level of theory and practice, and this should be utilized. the advanced workers should lead the less advanced workers.
And, since the state is in their hands, couldn’t they use it to serve their material interests, rather than the material interests of the working class?
this isn't a crazy concern though, we are aware that this is a possibility. this is just the nature of the state and class society; as long as these things exist, then there will be centralized power, and this can happen. if the vanguard becomes isolated from the masses, then it can rot. we have seen this happen, but as capitalism is always changing, marxism is as well, and we have developed new methods in order to combat this. specifically principles like the mass line are invaluable in these issues. the vanguard must walk hand in hand with the masses, it cannot march ahead of it and command the masses, while it also cannot tail behind them and be commanded. the two must form and maintain a mutual relationship.
nonetheless, this is not something that we choose to engage in, this is the nature of society; if we like it or not, these are things we will be forced to deal with until class is abolished. its not simply a matter of choosing a state or not choosing a state. a state will be forced upon a revolution if you like it or not, no revolution has existed without some sort of centralized power, nor can it exist without one. even non-marxist projects were eventually forced, by the material conditions around them, to adopt a centralized state. the CNT for instance was forced to adopt a labor camp system, they had forced collectivization, etc.
instead of looking at the revolution as a static thing, look at it as a kinetic thing. imagine the revolution as a road which we must march down towards communism. we are not going to complete this march in one attempt, that is not possible, it will take multiple attempts. we will be stopped, reversed, delayed, etc, but each time we get closer and closer. this is only done by learning from previous attempts, but learning from the attempts that got the furthest, and objectively those attempts that got the furthest were the marxist-leninists.
they didn't reach the end, obviously, they had a number of serious errors, obviously; but they got the furthest, and we must learn from this. we must take the correct and incorrect lines and analyze them, so that they can inform our next attempt.