Immutable systems do not "lock you out of changes you might want to do that aren't official", even if steamOS specifically does.

Microos and coreos both allow package layering client side. There is also features landing allowing "unlocking" the read-only filesystem.

More importantly, coreos now allows using Containerfile to customize the distro server side, and then all your computers can pull the same image (with all your hard work stored in the registry, and reapplied via the Containerfile and some cicd). It's certainly a very different and new workflow and there is a lack of tools user facing tools, but that is always the case with new tech.

The workflow I would sell you on is this:

• "unlock" system (allowing non-persistent changes)
• test everything works as you would like
• apply same changes via Containerfile
• Enjoy all your computers automatically updating to this and all your hard work being stored permanently in a registry.

This is a simplification and in practice currently a lot of things are hard to accomplish this way (say bootloader changes). But that is the "goal", and after running highly customized distros for a decade, I at least would love to be able to have all my changes in all my (present and future) computers without the hassle that brings currently

At least in coreos. rpm-ostree let's you "layer" packages on top of the base image, so when you install the next update it will automatically install your packages on top. You get to have the cake and eat it too.