One thing I don't understand about ranked

Whenever I call mid as a lower pick, and someone higher pick says " I'm going mid", I tell them " Ok, I guess I can go top or something, but if my performance isn't top notch, please don't blame me because mid is my main role." Then they reply " Learn to do all the roles before you enter ranked feggit."

If that is so, then why don't you go to a different lane and let me go to the lane I wanted? Since you know how to do all the roles and you're so good at them, why don't YOU cover for me?

That's the thing I don't understand about ranked. People say you need to know all roles before you start playing ranked, but then don't follow that rule themselves. This statement is so hypocritical. If YOU can do all the roles, since you're the one saying you should be able to do them all, then why is it so necessary that you NEED to go mid? Why can't you go adc for the guy who doesn't want to, and says he's bad at it? Please don't make this statement unless you yourself know how to do all the roles. Dear, Second Pick