First, colleges, at not the top ranked research schools aren’t about teaching. They’re about research. You’re expected to learn in your own, else why else have competitive admissions?
Second, my wife’s teaching degree, though supposedly highly ranked, was a joke, coming after her degree in natural resources. She was cruised by with a 4.0, and even so, was constantly dinged for things like not using enough colors in sticky-note bookmarks or highlights, not highlighting enough, etc. It was an absolute joke. She absolutely didn’t learn to teach either.
Can anyone teach? No, but a college degree isn’t the metric either. It is however a barrier to entry that keeps poorer people from joining the profession and also reinforces that institutional accolades are the mark of success, when really they are no such thing - they are an indicator of future success. Requiring four year degrees results in watering down degrees and erasing such indicators.
Both are important at all ages, but (A) seems more important at younger ages, where you need to know how to engage children. Kids are also more impressionable. (B) seems more important in middle school and high school. You wouldn't, for example, want someone who doesn't have a background in math to teach calculus.
A 4 year degree certainly isn't a perfect way to impart teaching competence, but it does set a threshold for the experience that a new teacher has. I'd expect more teachers at the back end of the bell curve if you remove the degree requirement.
How much depth is required to have a deeper understanding of anything than a 7th grader? How about 2nd grade?
Have a certification for the subject if you want. College is overkill.