Maybe your eyes have detereorated a bit after watching the C's for 50-odd years? Because I can tell you as someone who actually played AAU ball and against future NBA players, and got on the court mainly for defense, Kyrie's defense is fine. If you play PG in 2017, with more elite PGs than any other time in history, you're going to get beat sometimes.
Kyrie's defense is not simply fine, it's exponentially better than Thomas', even ignoring player size. Irving is actually able to stay in front of players and crowd the passing lanes -- something Thomas was shockingly bad at.
I am not ready to bash IT like that, the majority difference is still their size
I would expect a player like him to be able to compensate the lack of size with foot speed, but he appeared to lack both. You might want to chalk it up to his bad hip or increased offensive burden, or both -- but I thought the absence of instincts and inability to contribute to the defense was glaring. That wasn't to say it was due to lack of effort -- but sometimes effort can only get you this far.