Conventional foolishness of the wise men

A dialogue between two great Greek philosophers; Plato & Diogenes the cynic

• Plato was discoursing on his theory of ideas and, pointing to the cups on the table before him, said while there are many cups in the world, there is only one "idea" of a cup, and this cupness precedes the existence of all particular cups.
• "I can see the cup on the table," interrupted Diogenes, "but I can't see the 'cupness'."
• "That's because you have the eyes to see the cup," said Plato, "but", tapping his head with his forefinger, "you don't have the intellect with which to comprehend 'cupness'."
• Diogenes walked up to the table, examined the cup and, looking inside, he asked, "Is it empty?"
• Plato nodded.
• "So, where is the 'emptiness' which precedes this empty cup?" asked Diogenes.
• Plato allowed himself a few moments to collect his thoughts,
  
• but Diogenes reached over and, tapping Plato's head with his finger, said "I think you will find here is the 'emptiness'." (emphasis added)
  

Is Plato wise, in his idea of cupness? Or is Diogenes wise, in his assessment of vacuous thought?
The ideas of 21^st Century America are very Platonic, so, I appreciate Diogenes' ridicule of conventional "wisdom", yet, convention is not maleficent per se.
So, what is it exactly, in our postmodern conventions, which rouses my sense of "holy contempt", that is similar to Diogenes' contempt for the Classical Greek culture?
Are there empirical traits that one might perceive and quantify, which might allow a judgment of culture and convention?

I will consider this last question and maybe offer some opinions on which specific traits, agitate me to cynicism.