ではあるまいか

ではあるまいか puts forward the writer's own view as a question: "isn't it…?", "surely it is…". It follows a noun or a な-adjective directly; a verb or an い-adjective takes の first, and a な-adjective can take なの.

あるまい is ある with the literary ending まい, "probably is not". The question asks "would it not be…?", and the answer the writer expects is yes.

ではあるまいか or ではあるまいし

ではあるまいし differs by one sound and does nearly the opposite: "it's not as if…", brushing the idea aside. ではあるまいか puts the idea forward as probably true.

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ではあるまいか belongs to essays, editorials and speeches, and ではなかろうか is just as literary. In ordinary writing and in polite speech, use のではないだろうか; among friends, んじゃないかな.