Yes indeed, "smart kids and adults" can reach USCF 1800 with only minimum endgame knowledge.
After that, it's probably time to reinvent yourself -- largely because in the "A Class" and higher, players are increasingly strong with the black pieces and in the endgame, and your opening prep doesn't help nearly as much.
Many local OTB tournaments divide the Upper Section from Lower Section at USCF 1800. Most people don't realize that USCF 1800 is the 90th percentile of U.S. tournament players, and (more importantly), the upper 10 percent of players account for 50 percent of the tournament games played.
If your not playing OTB Games at 5-6 hour speeds, (which provides lots of time to calculate and move), then you better "imbide and know cold," lots of strategic endgame themes, othewise you'll invariably run short on time. This applies to all time controls of Game in 60/5, or faster.
Endgame knowledge helps take the "stupidity" and "blundering" out of chess -- because it helps you to play strategically, and effectively, at much higher speeds. You can play "faster and cleaner," with lots less mistakes. Indeed, this was the OP's original proposition for this thread, fully seven months ago. It still holds.

There was a group tournament once between endgame enthusiast vs opening enthusiast. Guess who won?
The midgame enthusiasts ?!