That would mean systemd entering package management territory. Now THAT would not be well received.

IMO, docker layering over the OS's built-in package management and update lifecycle in an incompatible ways is far worse than systemd replacing the init system and other service management functionality.

Back in the old days (late 90's, early 2k's) as a sysadmin I'd often write scripts to chroot or in other ways isolate services rather than run them as root, so extending the init system to handle those features feels like it's a logical extension, not a incompatible replacement.

systemd-sysupdate already exists. systemd won't run the software repository of course, but with systemd-sysupdate together with some overlay mounts you can get Steam Deck-like ease of use system updates.

For software management in R/W environments, there's the podman + systemd combo that'll let you run containers like normal systemd services.