Why Do Scholars Dismiss Plato's Model of Democracies?

Scholars claim that the actual data we have about democracies in ancient Greece refutes Plato's model. According to this data, they say, most tyrannies back then didn't evolve from democracies, but from oligarchies. Some of the wealthiest democracies were ruled by tyrants before they became democratic and not after. This suggests that tyranny was sometimes a stepping stone towards democracy rather than its after effect.