It's a ridiculous episode built entirely around a complete misunderstanding of evolution. There's no "ultimate destiny" for evolution, which is a mistake many sci-fi shows have made. The Valakians weren't destined to die to make room for the Menk; it was pure chance that they happened to develop a disease they couldn't cure. It's ridiculous that Phlox, a medical professional with decades of experience, would actually be the one to suggest and strongly support this moronic idea.
The episode was both poorly conceptualized and poorly executed (from a writing standpoint; I thought the acting was fine). It's just as absurdly overrated as "A Night in Sickbay" is underrated (not that I think ANiS is all that great, but I don't think it's quite as awful as its reputation suggests, either).