It's not the case with 〜ではあるまいし
Grammar: Reason & Cause
〜ではあるまいし expresses a cause or reason. It follows a noun or phrase, and is used to say “Because it is not ~” or “Because you are not ~”.
In the case of expressing a reason, a criticism, warning, advice, or an order may follow after し. However, it’s often the case that the portion after し is omitted.
When following a verb, ん or わけ may appear before で.
The colloquial counterpart is 〜じゃあるまいし.
Further Reading
- 〜ではあるまいし｜日本語能力試験 JLPT N１ (edewakaru.com)
- 【Ｎ１文法】～ではあるまいし／でもあるまいし／じゃあるまいし (nihongonosensei.net)
