Do you believe that democracies still face the challenges that Thucydides and Aristophanes complained of in Athens, such as demagogues, elite influence, or poor knowledge among voters? Can you name some contemporary examples?

Yes.

Even though democracy in theory must be something almost completely free of influence, all the influence of the people who have more power in society is still great, and it increases as social inequality grows

In situations where the population is in favor of deconcentrating income and wealth, this will has not found support in the political arena. On the one hand, there are those who insist on denying the problem, stating that the growth of social inequality does not produce adverse effects for society; on the other, there are those who declare that nothing can be done about the matter, since the culprits would be the "market forces", neutral and impersonal economic mechanisms.