What's more, supporting upgrades this way means you need to support every previous state file format, and the more state and features there are the harder this is to do. (Seriously, it was a pain in the butt to keep track of.) systemd is developed primarily by and for Red Hat who avoid this problem by not supporting upgrades to a new distro release from within the running system - but most other major distros do and will likely be bitten hard by the issue.

Without support for this you can't even reboot cleanly after upgrading init or any of the libraries it uses, because init will keep old deleted executable open and make it impossible to remount root as read-only (an obscure quirk of Unix filesystems that basically only init developers need to know). You don't just need to reboot to upgrade init, you need to boot into a special upgrade environment that doesn't use the system's init, and Fedora does. That's why Fedora needs daemon-reexec - otherwise they wouldn't even be able to apply security updates to glibc without rebooting first. Compare this with kernel upgrades, which can be installed immediately and simply don't take effect until the next reboot.