Re: "su is really a broken concept"
Long story short: su is really a broken concept.
So it's not like su is suddenly broken - it's just that some specific
new use cases don't work properly with it.
A fair number of people got their backs up for the very reason that su
was described as "broken". One could, of course ask whether in fact it
is the XDG Base Directory Specification
that is the broken concept, for incorporating the notion of the only way
that one reaches the point of running as any given user account being
login. ("the user being logged in ... the user first logs in ... the
user fully logs out ... the user logs in more than once ... first login
... last logout ... a full logout/login cycle") Design a mechanism that
at its foundation and throughout takes no account of adding other user
account privileges into a login session with su, or indeed that
processes wanting to create "runtime" files might be set-UID, and of
course it will conflict.