This reminds of an episode in Dr. House. Where one guy was in a vegetative state and was deemed incurable by other doctors, but of course Dr. House heals him etcetera. Now I agree this is just a work of fiction, but I am sure things like this can and probably happen in real life. e.g.: Somebody is deemed unable to give consent to anything and his relatives choose to end his 'painful' live, but a treatment could help him get back to normal.
Long story short, I would say yes. But only in cases where it can be medicinally proven that there is no way to help them. Which means, that their disease has to be confirmed.
Of course we could find a treatment in the future, perhaps the very next day somebody gets euthanized, but I suppose that is inevitable with decisions like these.
Cryogenic freezing till a cure is found?
Almost the same as euthanasia except you have a very very very small chance of coming back to life (when/if it is possible to defrost you and cure you)