No, I think "it's natural" is a bad defense of both.

Trying to prevent small children from squirming and getting distracted when they are bored and don’t have enough active outlet for their energy is pretty well impossible. Even harder if the kids don’t get enough sleep, aren’t eating enough or healthily enough, have to deal with strong emotional challenges at home, etc. Kids are on varied biological rhythms, and some times a kid is in a place where they simply do not have the physical capacity to sit and focus.

You don't need to make sure kids stay 100% still and focused 100% of the time. But you should reward good behavior and punish bad behavior. You should build up a work ethic and the ability to be productive.

The idea that you should stimulate kids to the point where boredom is impossible, or that we should excuse harmful behavior if the kid was bored / upset, is just silly. It only sets the kids up to fail; to live a life where they can't focus on tasks and they lash out whenever they are in a bad mood.

> you should reward good behavior and punish bad behavior

In general, you should not impose external rewards and punishments in education, to the extent possible. They block students’ focus, creativity, and problem solving, and undermine learning, literally interfering with memory formation.

This is a subject of incredible amounts of research in psychology and education, and has been demonstrated over and over in a wide variety of contexts.

> idea that you should stimulate kids to the point where boredom is impossible

Nobody ever suggested this. Only that class should be engaging and teachers should try to earn students’ attention instead of demanding it under threat.

> excuse harmful behavior [...] sets the kids up to fail

Nobody ever suggested “excusing harmful behavior”.

The proposal was that students who are wiggly or have trouble focusing should be thrown out of class because it is a “waste” for good teachers to teach them. I think this would be harmful, would be failing those students, and misses the point of education.

What sets kids up to fail is being treated as though they are worthless or being told so. Having access to good teachers does not “set kids up to fail”.