> idea that the US republic is the ideal institution

This is a straw man. Nobody, including the Founding Fathers, claimed the original Republic was ideal. The Constitution's vagueness and incompleteness were widely admitted at its signing.

Pure democracies don't work. They devolve into populist, majoritarian nightmares that tear themselves apart. This has been a consistent failure mode across over three millennia. We have minority-protecting, stabilizing features built into our system that have been systematically dismantled and attacked over the past decades, all in the name of more democracy. That more democracy isn't always a good thing has been absent from the discussion, in part due to Cold War and ensuing dynamics, in part due to ad hominem rejections like the one you made.

My counter to that is that the common act of holding the framers (and constitution) in reverence, which you did in the comment upthread, stultifies the ability of the country to react to changing mores and needs. There's a lot of absurdity in US law and politics that simply doesn't exist in countries with more modern and/or fluid constitutions.

Suppose that the US were to create a constitution today, based on today's needs and the anticipated needs of the next 50 years. How different do you think it would look from what was written in the 18th century? (Put aside the impossibility of doing this in today's political climate, as that's not the point of the question.)

I've gone back and forth on this. On one hand, the reverence is stifling and often mis-used. But it's also stabilizing. The fact that we've forgotten the balanced nature of our republic, and that the easiest way to remind us of that fact--which still holds merit today--is by appealing to the authority of the Founding Fathers, would be my Exhibit A to this argument.

How is that stabilising though?

It starts to sound demi-religious. You shall not disobey the word of a larger power/figure.

And then society starts to rift over time, through the interpretation by different sects about what the words meant.

Over centuries you get a pretty twisted view of the original values, interpreted by a few different sects, and an ever growing wedge between those interpretations that gets harder and harder to reconcile.

Just like most major religions.

They kind of did think they were building the ideal government, though. One of the major, under-emphasized influences on the framers (especially Jefferson) was Destutt de Tracey, who developed the term “ideology.” For Destutt de Tracey, ideology meant the formal study of ideas as scientific objects. Large parts of his writings deal with determining the scientifically optimal form of government (cf. Marx). In his correspondences with Jefferson, you can read how much the new government took from Destutt de Tracey, both in form and outlook, to the point that Jefferson felt compelled to explain deviations from the Count’s prescriptions.

Regarding the first half of your comment about instability and planning in the founding of the US, I've been reading a book that totally agrees and goes into more detail. Folks (like myself) whose only exposure to US history is public school classes and modern talking points might like it.

It's called Empire of Liberty. It's about the first 3 decades of the US and definitely shows how much was not planned at the time and how much of the constitution was intended as just a starting point to be edited soon after. Very much not the perfect document people treat it (knowingly or unknowingly) today.