Yes, the Founders’ chief and perhaps blind spot was failing to imagine Congress would give its powers away to the Executive Branch with both hands. Otherwise, the Founders did astonishingly well.
That might have not happened if we hadn’t passed the 17th Amendment and turned Senators into publicity seeking political careerists. In the older system States through their appointed senators would not have wanted to give up their power & influence. One the 17th was passed the state as an institution could be ignored. After the 17th a senator was now nothing more than just a congressman-at-large. The state corporately meant nothing it was just a corral to keep voters in.
I wish I still had this but some FReeper researched and show that prior to the passage of the 17th Amendment there were very few multiterm senators. Probably the reason being that state legislators saw themselves eventually in that seat as did other state grandees, so there was state internal competition.
George Mason and Patrick Henry both spotted the threat of a too powerful executive and they opposed replacing the Articles with the Constitution.
Go read what the Anti-federalists said and you’ll see that they were prophets