[beagle3]

> This doesn't fully explain why a reboot is not required on Linux.

Of course there is a theoretical possibility that this will happen; however, in practice, updates (especially security updates) on Linux happen with ABI compatible libraries. E.g. on debian/ubuntu

apt-get update && apt-get upgrade

Will generally only do ABI compatible updates, without installing additional packages (you need 'dist-upgrade' or 'full-upgrade' for that).

Some updates will go as far as to prevent a program restart while updating (by temporarily making the executable unavailable).

Firefox on Ubuntu is an outlier - an update will replace it with one that isn't ABI compatible. It detects this and encourages you to restart it.

All in all, it's not that a reboot is never required for linux theoretically - it is that practically, you MUST reboot only for a kernel update, and may occasionally need to restart other programs that have been updated (but are rarely forced too).