You don’t need to anticipate societal shifts, you need to empower the political system to handle them.

As for the longevity of the U.S., there’s a strong argument that it has in fact not lasted 250 or so years, and that the reconstruction era amendments created a qualitatively different, more centralized country. But even if we ignore that, the U.S. has a lot going for it that has nothing to do with politics. Two oceans, peaceful neighbors, and more natural resources than we knew what to do with that we bought for practically nothing.

The US emerged from WW2 as basically the only advanced economy that wasn’t flattened, did essentially zero introspection and assumed our politics were superior. And yet we have open bribery of politicians, and the highest court opened the floodgates for the wealthiest to donate to politicians. The only thing left is to allow politicians to keep donations for personal use.

Europe has surpassed us in life expectancy, and soon China will surpass us in GDP. And no matter how much free speech we have, we can’t have a conversation about it because the media is weaponized to distract us.

It doesn't quite follow that you highlight the need for a political system to handle shifts, then suggest that amending the constitution is not such a form of flexibility, but a transformation into another country entirely.

Normal amendments are fine. The 13th, 14th and 15th amendments weren’t just any old amendments though. They were passed via a non-political means (or if you take Clausewitz’s opinion, that war is a continuation of politics by other means, it is still a non-constitutional means), effectively without the southern states’ consent.

Almost everyone today thinks the reconstruction amendments were good, but the fact that we had to fight the bloodiest war in our history to get them is not a compliment to our political process.

Europe's lifestyle is subsidized heavily by the American defense industry and they had better buckle in if Trump wins and starts more seriously winding down America's defense commitments there

Isn't it a benefit to the US for other countries to have weak militaries and be dependent on the US for defense?

There is approximately zero chance Trump would redirect military spending towards healthcare for regular people. His main accomplishment last term was a trillion dollar handout to corporations. He doesn’t represent you and me.