Well, maybe no one actually said the words "hey, the natives might still find a cure without us", though I kinda suspect that someone did, but I'm pretty sure that no one said "they absolutely won't find one", either. Just because the natives hadn't found a cure yet, and had sent people into space as an alternative venture, doesn't make them "incapable" of doing so. (Nor, for that matter, does it preclude the possibility that another spacefaring race might decide to help them.)
From the episode, next-to-last scene:
Archer to Esaak: Phlox tells me this medicine will help ease the symptoms for a decade, maybe more. A lot can happen in that time. I wouldn't be surprised if you developed a cure on your own.
As for the eventual fate of the Valakians, Phlox talks about "evidence" of this, "potential" for that, "opportunity" for the Menk, and his talk of Valakian extinction is "based on my projections," which to me sounds like he's mostly sure, but not 100%, because he's a doctor, not a soothsayer.
Nor does anyone say the Valakians will never find a cure. All we know
is, they get medicine that helps to sustain them, and they seem sure-fire determined to get their hands on a cure by any means necessary and don't seem to be planning on giving up.