Disclaimer - I've never flown a large scale passenger aircraft but have significant experience in light twins - passenger/cargo/aerial survey.
Never had the luxury of a second pilot
Never had the luxury of a ground engineer to sign the aircraft off each day/flight
Never had the luxury of someone with whom to discuss weather/technical diversions
Never had the luxury of ground crew support
Never had the luxury of a call button for coffee
You know what?
Never had a problem flying single pilot. Automation is significantly less in an old GA twin - forget autothrottles, often the autopilot doesn't work!
I've done multiple day after day 6 hour stints doing lidar aerial survey (where you can't exceed 1 degree angle of bank and more than 6" of lateral deviation - it's pretty demanding manual flying) then ended by flying half an hour to the alternate and shooting an ILS to minima (on raw data and without an autopilot).
It's not hard - personally I think it's fun. I'm not special, I'm not bragging, there are a lot of pilots who do exactly that everyday. There are even the girls and boys of the RFDS that fly single pilot to unlit strips in the middle of the GAFA in the middle of the night - now THAT's special
Are we seriously suggesting in a highly automated modern aircraft that is designed for single pilot ops, just because there are more than 9 pax, you need a second pilot? I can see it for long trips (perhaps peeing into a bottle on the flight deck will never catch on in the airlines)
Single pilot/UAV is coming, just as soon as the SLF accept it.