docker got requests to allow you to configure a private registry, but they selfishly denied the ability to do that:
https://stackoverflow.com/questions/33054369/how-to-change-t...
redhat created docker-compatible podman and lets you close that hole
/etc/config/docker: BLOCK_REGISTRY='--block-registry=all' ADD_REGISTRY='--add-registry=registry.access.redhat.com'
For example, if you're on a team and you have documentation containing commands, but your docker config is outdated, you can accidentally pull from docker's global public registry.
A welcome change IMO would be removing global registries entirely, since it just makes it easier to tell where your image is coming from (but I severely doubt docker would ever consider this since it makes it fractionally easier to use their services)
Even if you could configure a default registry to point at something besides docker.io a lot of people, I'd say the vast majority, wouldn't have bothered. So they'd still be in the same spot.
And it's not hard to just tag images. I don't have a single image pulling from docker.io at work. Takes two seconds to slap <company-repo>/ at the front of the image name.