でなくてなんだろう

A rhetorical question meaning 'if this is not X, what else could it be?' The speaker emphatically asserts that something is truly X, often with strong emotion or admiration.

A literary, emotionally charged expression typical of essays, speeches, reviews and narration, where the speaker praises or passionately characterizes something as the very embodiment of X. It can also be spoken for deliberate dramatic emphasis. It is more dramatic than a plain assertion such as にほかならない, and much stronger than a casual rhetorical ではないか.

• 1.
  にほかならないN2「にほかならない」 also asserts "it is precisely X," but with a calm, expository tone; it can also assert an action with 「verb＋こと」 and stress a reason or purpose with 「から」/「ため」. 「でなくてなんだろう」 is a rhetorical question charged with deep feeling and attaches only to nouns.
• 2.
  というものだN2「というものだ」 makes a direct assertion without a rhetorical question, is neutral in register, attaches to nouns, adjectives, and verbs, and is often used in criticism. 「でなくてなんだろう」 attaches only to nouns and, with its lyrical written style, carries even stronger emotional weight.
• 3.
  とはN1「とは」 expresses surprise at something unexpected — its core is "I never expected this" — and attaches to verbs, adjectives, and nouns, whereas 「でなくてなんだろう」 asserts "this is precisely X" through a rhetorical question and attaches only to nouns. Both lean toward written style, but they are not interchangeable.
• 4.
  ではないかN4In its "isn't it...?" sense, 「ではないか」 is likewise a rhetorical assertion, but it mainly conveys discovery or surprise while seeking the listener's agreement, and its emotional force is milder than this pattern's. It also attaches to adjectives and verbs and has separate uses for conjecture and invitation.
• 5.
  でも何でもないN1「でも何でもない」 is a strong denial meaning "it's not X at all" — the exact opposite of this pattern's strong affirmation — and is mostly used in speech, often with a dismissive tone. It can also attach to na-adjective stems and to 「〜くも何ともない」, so the two are not interchangeable.
• 6.
  に決まっているN2「に決まっている」 asserts "surely" or "of course" based on common sense or subjective judgment, attaches to verbs, adjectives, and nouns, and is mostly used in everyday conversation. This pattern instead expresses deep feeling through a rhetorical question, attaches only to nouns, and is formal and written, so the two generally cannot be interchanged.

にほかならない and というものだ state the same kind of conclusion flatly, and に決まっている expresses conversational certainty, whereas でなくてなんだろう makes the claim through an emotional rhetorical question. ではないか is a lighter rhetorical appeal, でも何でもない denies that something is X at all, and とは expresses surprise or introduces a definition.