Creating further barriers to enter teaching might make sense in order to make teaching a more desirable career in the long-term, but it could easily create even more artificial scarcity in the near term, risking an even wider teacher shortage.
For all the difficulty of doing a rigorous evaluation in order to improve teaching performance across the board, I suspect it is a whole lot easier to identify the worst teachers in the school.
Lumbly makes you believe that this Johnny Appleseed of reading and writing would return to slavery from a free life up North and risk mutilation in order to continue his teaching.