Trying to honor the user's customization of 'completion-styles' makes
a certain amount of sense. Though I don't know how much we honor it this
way: if the user is relatively new, they might not even know to keep typing
to see the fallback, after noting that their input does not give them the
matches they expected.
Actually, I suspect it works better for new users than for old ones: new
users don't yet have a clear mental model of how Emacs's completion
works so they might expect something more like what you see with a web
browser search where adding more data can completely change the
proposed completions.
In contrast old-timers may indeed "know" that there won't be any
completions further down and will never reach the second style (to some
extent, that's how I got Richard to accept `partial-completion` in the
default).
It's more of a critique of the whole "list of styles" design, admittedly.
I don't regret doing it because I don't think there was any other
way to activate `partial-completion` by default, but yes it has
its downsides.