I think the default should just be group if you are in one. There is never any harm in saying it into group channel "wrongly", because you can w/c right away (no chat cd) and it doesn't consume any resource (trumpet).

Whereas defaulting into world can be.... problematic.... (besides, whatever you want to say to world, your group can see too, so no "secret" is lost. The converse is not true.)
Might be easier to implement than a lock