I find this phrase rather demeaning. I am a damn good Instructional Designer, but I would eventually like to teach this to others.

The saying sucks because it implies that they tried doing something in the private field and failed into teaching. Most teachers went directly into teaching, so they succeeded.

It’s a dumb saying. Teaching is harder than many jobs

Usually said by angry types who never did either one.

Yeah that's so out of context that it speaks to the intelligence of students repeating it. It came from the old trades and people retiring into teaching after they literally couldn't do it anymore, not that they weren't once capable. Very similar to the "Bad apples" quote.

Wow, what a stupid thing to be around.