If material conditions are all that shapes ideas, why are you speaking ideas at me? Either you are incorrect or you are wasting your time, and I don't think you're wasting your time.
In a world where ideas don't matter, either of us working against our class interests through ignorance is incoherent. If we can be deceived to act against our class interests to change material conditions to serve the rich, then ideas can change material conditions.
People can interpret their interests different ways. People can ally themselves with specific groups but not others. People can have different levels of risk aversion. People can be affected by propaganda and charisma. Ideas skew praxis skews the development of material conditions.
Neglect this at your own peril. Use it to make fairy tales come true.
To conflate proletarian dictatorship with bourgeois state power is to erase the question of class content.
What class attributes do the authorities in the vanguard have in common with the proletariat?