Thanks for researching this! The /etc/mysql/debian.cnf file is only root-readable, and intended for root users to be able to access the database console e.g. when the maintainer scripts run and start/stop the server. The new way of using socket authentication is a superior way from both security and management point of view. The file has been obsolete for a couple of years and now I emptied the contents of it.
According to codesearch.debian.net there are quite a few places that run `mysql --defaults-file=/etc/mysql/debian.cnf` and they will fail if the file is missing, so it is now there, but empty. I did not find any code that would change the username and for recent years the setting has said "user = root", and before that it was "user = debian-sys-maint" One option could potentially be that if dbconfig-common does not find the username, it could default to just "root". I will wait for a couple of days to see if anybody else sends feedback about /etc/mysql/debian.cnf and then decide what to do.
