I think their caution was somewhat warranted, given the relative novelty of the Republican experiment. They were fist and foremost concerned with keeping the union from collapsing, and were horrified at how the French Revolution turned out. They knew perfectly well that conditions would change, which is why they made a system to amend the constitution. If anything, they expected more change, but didn't know just how much entrenched economic interests would mold politics. And to the extent that they did, they didn't have any way of preventing it, at the time.