True as far as it goes, but is really likely that men, elves, and orcs (really all but hobbits) could have that many thousands of years of civilization at a stable or declining level of technology and magic, with so many wars and disruptions of bloodlines, without trying out any form of government other than a kingdom?

Yes, actually. Look at the history of say China before major Western contact, or Japan, or India, or Mesopotamia, or Ancient Egypt, or really anywhere outside Europe or extremely heavy European influence.

I know elves are stubborn,

More importantly they're immortal.