〜でなくてなんだろう
if this isn't X, what is?
Noun + でなくてなんだろう / でなくてなんであろう
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でなくてなんだろう is a rhetorical question insisting that something can only be X: これが愛でなくてなんだろう, "if this isn't love, what is?".
The noun is a strong, abstract word: 愛, 奇跡 ("miracle"), 運命 ("fate"), 差別 ("discrimination"), 犯罪. The speaker is moved or outraged, and the question has no real answer. It means "this is X, without a doubt".
It's literary and dramatic, found in essays, speeches, novels and song lyrics. でなくてなんであろう is even more formal.
Don't confuse it with ではないだろう, "probably isn't", which is a genuine guess.
Examples
これが愛でなくてなんだろう。
If this isn't love, what is?
全員が助かったのは、奇跡でなくてなんだろう。
Everyone survived. If that isn't a miracle, what is?
二人の出会いは運命でなくてなんだろう。
If their meeting wasn't fate, what was it?
見た目だけで判断するのは、差別でなくてなんだろう。
Judging people on looks alone. If that isn't discrimination, what is?
こんな偶然が重なるとは、神の導きでなくてなんであろう。
So many coincidences at once. What could it be, if not the hand of God?