Hi orda,
I agree with you until you get to the inevitability of failure.
It may be that the man-made construct which is the US does not have longevity. It may be that it does. My contention is it will have more staying power if it is capable of continuous adaptation. Hari's (if I may) is that it will have more staying power if it is protected from temporary error. Ideally both these things are true, but if it is a choice, I'd risk the errors of temporary whimsy as I believe that a system which is obstructive of gradualism is one that occasionally needs to shift seismically - and punctuated equilibrium is a system which is inherently unstable and randomly destructive in my view.
If you want an example of a democracy that has longevity and that seems to work and that is built upon property rights, there are several examples. Usually they have geographical coherence. It seems to me that the US is more of an empire in which the constituent states have disparate interests. These interests will express themselves over time. So I doubt the unity's longevity. Whereas many of the individual states seem robust.