> And theoretically talented grads would make great teaching assistants, but those are exactly the people who get much better jobs so they end up with students

> And theoretically talented grads would make great teaching assistants, but those are exactly the people who get much better jobs so they end up with students who struggled through the program "teaching" the next couple cohorts. You nailed it. It's the complete opposite of someone who was a TA during undergrads.