Hello, and thanks for your question! My name is XXXXX XXXXX X am happy to assist you today!!
You are correct in your idea that they are actually plated
Most kick plates are actually plated because if they were actually solid brass they would be to soft and would become damaged easy. So they electroplate a cheaper sheet metal in brass that is only a few microns in thickness.
Your best bet will be to replace them if you want that shine back.