〜でなくてなんだろう literally means : "If it is not 〜,then what it is/ what can it be?"
well spotted. Level 1 i think.
JLPT Level 4?
JPLT Level 1 without a doubt.
not to be a nit-picker, but how does that example sentence fit in with "written only"?
suppose we're talking about a person, is 〜でなくてだれだろう acceptable? (same goes for places, time, etc)
たとえば？あの人はスミスさんでなくてだれだろう？とか？I think its grammatically correct its just probably not used. Also consider that when speaking about other people you have to be careful about politeness, so saying なくてだれだろう？might be considered a little bit rude.
Is there an equivalent word/phrase which means "if that isn't (X) then what is?" or similar? It'd be very handy to use in day to day life.