I think humans can definitely return to Earth in some capacity. Iori can eat the food the staff cooked just fine, so her immune system did not deteriorate to a point where she could only survive in a spaceship environment. Allergies are not deadly viruses - even if they can't be cured, they can be managed with antihistamines and such. And if humans can build colonies in space, building one on Earth would be far simpler, with more resources and space to go around. They will eventually adapt, for sure.
Also, I sure hope Yachiyo is not some lost technology, even though Iori's reaction to her certainly implies so. I hope other robots can be fixed.