I chanced on this article which raises the titled question: Why aren't two pilots mandated for all planes esp. military? What happens if the lone pilot becomes incapacitated, esp. if the plane has passengers who aren't pilots?
Two captains of Cathay Pacific Airways lost sight during two separate flights, according to the Hong Kong Civil Aviation Department. In both cases, the first officer managed to land the aircraft safely.
On January 28, 2016, a pilot of the British Royal Air Force had lost sight during a training flight in a Hawk jet in North Yorkshire. Another jet had been scrambled with an instructor on board that helped the impaired pilot to land at RAF Leeming air base. Sources quoted by the Telegraph had said at the time that his vision was affected by the sudden deterioration of an eye infection.