Furthermore, Millet systems are extremely unstable, to say the least. There being a reason there’s no longer any Ottoman or Austro-Hungarian empires.
I take a different lesson. The Ottoman Empire and Austro-Hungary (if you include its de facto predecessor the Hapsburg Empire in its longevity) were extremely stable. Their institutional lives extended for ~500 years (lopping off the initial periods when both state systems were compact). The American system is only a few decades over 200, and it is has the longest continuous history as a liberal democracy in the world today.