If that's the case then why do people raise concern when they noticed that a user changed his/her email? If nobody cares, then account selling will continue on a steady basis.
If there is no strict restriction on this issue, people will continue to sell accounts while claiming they only changed their emails or passwords.
I've seen some threads discussing the implementation of 2FA in forum accounts, which I believe will help to reduce quick and immediate account compromise, as each user will use the 2FA security to strengthen their account security.