Is the statement of one who says that if the pancreas fails in a diabetic, medications are useless for completely curing it, but rather make him coexist with the disease, considered forbidden for contradicting the hadith: "Allah has not sent down a disease except that He has sent down a cure for it"? And is believing this statement considered disbelief for contradicting the hadith?

There is no room for disbelief in what was mentioned, and there is no contradiction between describing an illness as chronic or incurable, and the prophetic tradition: "Allah has not sent down a disease except that He has sent down a cure for it," because the Prophet, peace and blessings be upon him, followed that by saying: "known to whoever knows it and unknown to whoever is ignorant of it." Thus, the description of chronicity is within the knowledge of doctors and is not a description of Allah's decree. Al-Hafiz Ibn Hajar was of the opinion that a fatal disease for which doctors have admitted there is no cure falls under the general meaning of the hadith, and it is possible that the reference in "unknown to whoever is ignorant of it" points to that.