If what you are saying is true, why is Tesla charging ~$2200 + labor to replace MCU1 units out of warranty? If these refurbished MCU1 units are so cheap then they must be ripping people off, especially since they are keeping the original unit so they can refurbish that one.
You answered the question yourself. Almost every failure of the MCU1 is due to the failure of the relatively inexpensive eMMC chip that Tesla uses.