In fairness, deleting (even moderately) old stuff from /tmp is a Unix convention that goes back roughly three decades prior to the very existence of systemd. In SCO Unix in the early 1990s, for example, it was the task of the /usr/lib/cleantmp program. AIX had a script, invoked by a daily cron job, called /usr/sbin/skulker that did this and more besides.
Putting stuff in /tmp and expecting it to stick around has been a user and indeed a system administration mistake for all of that time, motivating, for example, warnings like this in Kent's & Williams' 1990 Encyclopedia of Computer Science and Technology (volume 22, "The UNIX operating system"):
> /tmp and /usr/tmp: These directories are used by utility programs to contain temporary files. They are cleaned out periodically, so files should not be stored here.
Que Corporation's Using UNIX and Using Linux both said:
> Because the system automatically deletes the contents of this directory periodically, do not keep anything you might need later in this directory.
15 years before the existence of systemd, Linux Filesystem Structure (later to become the Filesystem Hierarchy Standard) was similarly warning:
> /tmp may be cleaned out at boot time or at relatively frequent intervals. Therefore, data stored in /tmp should not be expected to remain for any long period.
A mere 13 years before the existence of systemd, that warning was made starker:
> Programs must not assume that any files or directories in /tmp are preserved between invocations of the program.