Meaning of「残るは」in a sentence

Question

Came across this sentence in a memoir and I have a grammar question about it. For context, a person is thinking about someone sleeping nearby:

さっき声をかけてきたヤツは当分寝ないだろ。残るは寝ているか寝ていないかわからない隣の隣。

Regarding 「残るは」, is this a set phrase or expression? It's not a nominalized verb, as far as I can tell, as those typically possess the particle の, or こと. And I've seen it pop up in other things I've read. I understand the general meaning as:

The guy who spoke up earlier won't be asleep for some time. That leaves the guy next to him（隣の隣）, who I'm not sure is asleep.

Yet why is 残る left with は, as opposed to こと or の? Is it functioning in a similar way to には after a verb, whereby it doesn't need to be nominalized with こと or の? Given the very modern context of the memoir I'm not convinced it's a case of classical Japanese slipping in, but it might be a fixed expression used more generally in certain situations? It's hard to say.

Answer 1

For me this sounds like a shortened version of "残るのは"。 Usually the noun form of "残る" would be "残り" or "残るのは" etc, so this would be a rather rare(old-fasioned) and literal expression. According to the "related post" below, this nominalization occurs in limited cases (most often in a proverb).

Similar expression might be "逃げるは恥だが役に立つ", here "逃げる" is a noun meaning "to run away", which is normally expressed as "逃げること" or "逃げるの". There is also a saying "聞くは一時の恥、聞かぬは一生の恥", meaning, asking is a shame for the moment, and not asking is a shame for lifetime. (Although it is different, in a sense, "残る" here means the one remaining and "聞く" means the action of asking. )

I don't have an answer for whether this expression is similar to "には", sorry about that.

related post in Japanese: https://detail.chiebukuro.yahoo.co.jp/qa/question_detail/q12169436772