The reason we even got a constitution is because—as toxic and terrible and selfish and monstrous as they were—everyone involved had the self-awareness (even Hamilton) to admit that they might be wrong. That their task was hard and they might make mistakes. They committed TO FIXING IT.
The Bill of Rights only exists because during the ratification process everyone agreed that it needed some amendments! The Constitution only got ratified because they promised a Bill of Rights! The Founders are the ones who changed the Constitution the most! And they expected we would do more.
I’m going to chalk that up to postwar naivety. They had just beaten the largest empire on earth while practically starving to death with no resources; they thought they had transformed human nature.