That's a very good argument, but people have down used to dropping the domain prefix because it's always been optional.
Giving people the option to configure a default repository via the daemon.json would alleviate that issue, but I'm not sure if that's really enough to fork.
Oh great, so now it's a hidden default that might be different system-to-system. Disjoint from the actual deployment config. Please, dear god, no. I'm sorry, this is a bad idea.
The web is absolutely littered with docker tutorials and a huge proportion of them (not operated or maintained by docker themselves) would no longer be valid, I'm sure.
Giving people the option to configure a default repository via the daemon.json would alleviate that issue, but I'm not sure if that's really enough to fork.