> Steel-manning the idea, perhaps they would ship object files (.o/.a) and the apt-get equivalent would link the system? I believe this arrangement was common in the days before dynamic linking. You don't have to redownload everything, but you do have to relink everything.

This was indeed comon for Unix. The only way to tune the systems (or even change the timezone) was to edit the very few source files and run make, which compiled those files then linked them into a new binary.

Linking-only is (or was) much faster than recompiling.