The US Constitution is an early prototype for the constitution of a modern republic. Pretty much every other constitution in the world has been written based on US experiences. In almost every case, the newer constitutions have chosen to organize the republic in a fundamentally different way. While you may argue that the values in the US Constitution are the best in the world, it's nowhere near the state of the art in procedural issues.

A key test for a constitution is whether a substantially similar constitution would pass the amendment process today. If it wouldn't, the constitution is obsolete and should be rewritten. If people no longer believe in the spirit of the constitution, they will increasingly try to subvert it. Institutional checks and balances may protect the constitution for a while, but if people don't believe in the constitution, they probably won't believe in the institutions either.