Knees do not necessarily wear out evenly, sometimes one part of the knee is perfectly fine while another part is completely destroyed. If only part of the knee joint is worn out, why replace all of it?
Exercise for fun, or for training, or both. Exercise wisely, in ways that help you in sports or your daily life. But don’t do exercises that are useless, or potentially injury-generating. Here are a few doozies that should not be on your list: