Where are the descendants of Plato?

Nowhere. Plato was a homosexual who never married nor had children. He was member of Athens most distinguished family - where are they? Nowhere. They left no issue. The current population of Peloponnese carries the African gene against malaria, suggesting that the natives were semi-extinguished by malaria. The same is true of Sicily and even Rome.

The people found in Roman Cemeteries are mostly North African and East Mediterranean - few original Latin people. The malarial mosquito appeared in those areas during historical times and caused a profound population replacement. So we have an historically documented dying-off of the Classical Greek elite, and of Roman Senatorial families. If I remember the numbers, from the original 250 senatorial families none remained by the third century. Roman senatorial families did not necessarily ended for lack of children as adoption was widely practiced.

Most astonishing, the whole Greco-Roman world disappeared and its densely peopled cities were abandoned and forgotten. All that was known, over 95% of the books, was destroyed. Science had to be re-invented in the Renaissance starting in Northern Italy. On the other side of continental mass, on the Pacific sea board, the Chinese maintained uninterrupted civilization. What we learn from history? that it can happen again.