It's a mass noun just because the adjective turned into a noun (sorry, don't know the technical terms in English) when the real noun was elided. In "the poor p

It's a mass noun just because the adjective turned into a noun (sorry, don't know the technical terms in English) when the real noun was elided. In "the poor people", "poor" is an adjective. If you elide "people" and turn the phrase into "the poor", it has to be a noun (otherwise we'd have a noun phrase without a noun). Though IMHO it's better analyzed as an adjective with an elided noun, but I'm afraid I lost that war ages ago :shrug: But I personally prefer to analyze what makes sense, not what's written (considering language is language). I'd rather have a regular case with an ellipsis than "oh, it's just a fixed phrase" and a zillion other corner cases.