Okay, the natives were shown to be incapable of finding/creating a cure on their own.
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.)
Remember that our
discovery of penicillin was a complete accident. Science isn't a steady process; it advances in leaps and bounds. With a whole planet full of individuals working on the problem, who knows what might happen?
And, since the situation was never referred to again, TighsEye
's assertion that the natives didn't find a cure is simply unfounded speculation.