I read that systemd developers would rather have offline installations occur at startup when the environment is known rather than at shutdown, and I understand that.

I think that doesn't prevent from offering this set-up: instead of normally shutting down, reboot to the offline install target, and once the offline install is performed, instantly shut down the machine.

that would offer the "install at shutdown" offer that is considerably more handy to many people, me included. that is what MS Windows offers and when we don't know how long updates may last and really have no time to waste waiting for them, it's really the most practical for most users I feel. Let the computer deal with that while we don't need it.
Actually the question might be, is the user letting the computer on all day long, or shutting down when they don't need it. If the situation is the latter then I believe this new option would cater for the need of those users considerably better.