now you're talking about a different issue. But as evidence that I'm on the same page with you on that: https://www.cougarboard.com/board/message.html?id=19705381
The only thing holding back the online model is the ability to motivate students to learn on their own and their competence at finding knowledge on their own. Well, and you have to get regulators to allow it.

But back to the "ceiling" you were talking about before. Let's hold everything else constant for now, is there a limit to how much knowledge can be gained in primary and secondary education like there's a limit to how well a burger can be made?

If I can get students to learn all of the K-12 content by age 14 (which I believe is very possible), then you can add that much more knowledge into years 14-18. Better teachers can not only teach faster and more efficiently, but they are are better at motivating students to learn on their own and care about learning. I literally see it every day as a university professor. Ask any student; there are faculty who can teach them anything and there are faculty who teach them nothing. More pay leads to better faculty over time.

My wife was not ahead because she was inherently smarter. In fact, she ended up dropping out of college; felt like it wasn't for her. She was ahead of me because her private school paid for better teachers and they were worth the pay. If you don't believe that's possible, then you don't believe in one of the fundamental assumptions of the free market system.