Having flown 15-16 hour international flights, I wouldn’t recommend having ONLY one pilot in the cockpit. Military tactical pilots often resort to GO pills for transoceanic flights. Even in the military, we flew multi-leg flights across the Pacific, all on the same day, to arrive 10 hours out of whack. I’ve flown Antarctic flights and Arctic flights with 24-hours of daylight when you realize you are just a big sunflower with the sun dictating your physiology.
Weather threats, altitude changes, frequency changes, border crossings, back side of the clock schedules, diversions, emergencies, time zone changes of 10 hours or more, systems failures, and non-homogenous pilot skill levels are just a few of the variables that interrupt those hours of boredom.
As a check airman in the 727/757/767/777 fleets for the last 20+ years of a 36 year career at a major airline, FLASH, not all pilots are created equal. That is not in the constitution. Even half of all doctors graduated in the bottom half of their class. Most guys and girls get it, a few never do, some are trapped in a financial rewarding career they can’t replicate elsewhere.
Fortunately, most of the pilot population never face a complex problem, just like most career law officers never have to discharge their weapon. The airlines deliberately do not pair up new pilots together, with the union’s blessing, Some new guys having problems are paired up with check airman for extended periods. I know of pilots who must sign agreements to never bid Captain or are demoted from a captain position. Cockpit Resource Management was a great development but a few dictators are always going to lurk in the weeds, you know, like other professions.