This makes me think of something that has been mentioned at various
times: couldn't we have a "default" version (which would happen to be
in english for core) and english as just one translation, which would
happen to be empty by default for core, returning the default version
?

In the past there has been severe performance implications when the locale
module was enabled. Sites suffered heavily from excessive queries.

I am not sure if this is still the case, but since all of sites I do are English,
I would not want to go that route for performance's sake.