Russia and China recently amended their constitution to ensure their respective leaders can stay in power permanently. Having some unchangeable bedrock in a country seems very valuable as we are all vulnerable to tyrants at some point in history.
What matters is that the society itself believes in the traditions of democracy, freedom and the rule of law. If that withers and dies, perhaps because people no longer consider these ideals to be as important as their pet ideologies, or because the citizens no longer share a common epistemological reality, or because oligarchs are given free reign to buy politicians, courts and media, then it doesn't really matter any more what your vaunted Constitution says.
US only got Presidential term limits several decades ago. Simply codifying a tradition after it was challenged. The term limits haven’t always been a net positive.
Something more modern to prevent tyrants could be helpful.
Also, you call the US constitution unchangeable? It's quite laughable. Not only there is a bunch of amendments, but real power structures have changed as well. Starting from the increased size and power of the federal government as opposed to what was envisioned by the Founding Fathers, following by the two terms restriction on presidents, and ending with the creation of the central banking system, which arguably goes against the spirit of the Constitution.
I see you're not a fan of Alexander Hamilton. The (first) US bank predates the other items in your list, though.