I fail to see how only requiring a high school diploma is going to make it easier for someone like a doctor to transition into teaching (why in the world would they anyway). If that doctor's credentials didn't qualify them to teach something related to their field what's the point?
I have a BS in Environmental Health Science and an MPH in Occupational Health and Safety. My entire career has consisted of me teaching people safe food handling, proper safety precautions and conducting inspections for both. I'm confident I could step into a classroom and teach public health now, but I could definitely benefit from a course on classroom management.
We need to look at "WHY" properly degreed educators are leaving their jobs in the first place. There you will find the root problem. Public secondary education has transformed into something that neither educates nor prepares young people to exist, let alone prosper, in our current world.