MY argument would be that the increased Federal power after the Civil War made the further abuses of power much easier. Sort of how HIV doesn’t actually kill the host, but renders the host vulnerable to a future infection.
"The original US Constitutional republic died the day the civil war ended"
Not "sustained injury". Not "infected". Died.
If the poster's argument is correct, then passage of the 17th amendment was irrelevant in 1913. It couldn't have destroyed or harmed anything in our Republic, since the Republic had already been dead and buried for decades.