I agree that allowing people to become credentialed as teachers as an after-the-fact procedure would certainly make it easier to find people qualified in the subject to teach it. I believe it would be better all around if people who want to teach got their undergrad in something other than education.

It could be done if the states' Departments of Education turn the teaching cert into a graduate level degree: icing on the cake of subject competence. This would include student teaching, the biggest barrier to people seeking to teach as a second career, because it obviously can't be pursued while working a day job. Also, schools can pay all or part of tuition for grad-level courses as state-required professional development credits, helping out the student/employee who probably experienced a considerable drop in income in the migration from most other professions to teaching. Student teaching is, as far as I know, offered only as an undergraduate course and is so both required for certification and ineligible for aid. Booo, hiss!

That said, let me tell you as a veteran of parochial schools, sometimes private schools (who don't need to hire credentialed teachers) will hire people who worked in an area rather than a teacher who was later trained (one hopes) in the area of instruction. Sometimes, especially if it's an upper-level class of self-selected students, the class can be wonderful. If it's an on-level or introductory class, which it would have to be if the thought is to excite students not already hooked, the results are mixed. Some people who are wonderful at doing their specialty couldn't teach their way out of a wet paper bag.

I'm thinking of my algebra teacher, a mathematician. Trust me. She eventually let those of us who got it, sort-of, go across the hall into a garret room to teach ourselves. (The high school building was a very large former orphanage.) Meanwhile, she struggled to understand the questions of the other students, and they struggled to understand her single replies. She didn't know how to explain more than one way. It was dreadful.

Enthusiasm and knowledge can't substitute for the ability to communicate. Some people are incapable of effective communication with anyone outside their specialty; some people who don't have a natural talent for it can be trained in the techniques. Teacher education will weed out the former and train the latter. Professional credentials really are worth something.