No one brought up valid use cases for docker-in-docker. They brought up the issue that gitlab mandates docker as an interface (which I totally understand, btw.).
For instance the "how do I compose multiple docker containers" is trivial when you can just execute a script that runs docker or podman. If you really want, you can use docker-compose.
You started out with asserting "you're too far into Docker", we bring up valid use cases for docker-in-docker, and then you saying "This herd mentality [..] is really fundamentally problematic" is really not adding a lot to the discussion.