Are there people who think, "Oh, I've only gotten 9,500 steps a day this week; my health will begin to fail." ?
10k steps a day is not a myth, it's a goal. X glasses of water a day is a goal. Most health instructions are goals, and most people will fall short of the specific targets. How short you fall is likely to have some impact on your health, but each person is different.
While being active is undoubtedly very important for health, it's very clumsy to focus on one behavior and highlight the difference in mortality risk. It's very likely that more active people have other habits that contribute (significantly) to mortality risk reduction.