As you said, the system was set up first and foremost to be divided between the feds and states. THEN in importance were three branches, followed by a BOR.
The last one hundred years show a BOR unsupported by the competing interests of the States within the larger government is just a parchment barrier . . . worthless.
All told, however, though it has been seriously altered in many ways, overall the BOR has proven essential in thwarting bad government more times than can be counted, because the systems of checks and balances are far less capable in doing so.
The evidence of this lies in Britain, that had no written BOR, and has suffered markedly because of it.
And, it should not be forgotten, that when the government does stray, the original document and the BOR exists as a reminder of what it should be, and a guidepost to what the government should return to.
For example, if you took 100 strong conservatives, and gave them a year to argue how to restore the country, they would likely not need a year, because they wouldn’t need to experiment with untried ideas. Instead they could refer to “what worked before”, and “why did we stop doing it that way?”, as their guideposts.
The results of this would be the discovery that when we stopped doing something that was right, it was done for pragmatic reasons that no longer apply. So there is no reason to continue the change, and we should go back to doing things the right way.
This in no way is “reactionary”, because a reactionary path is one that seeks a false, idealized former system. Instead we would be returning to a system we *knew* had worked, so most likely would work again.