My view on this subject is simple, why the h$ll not have a biz jet that can fly in the FL500 to Fl600 at M0.95 flown by a single pilot, there is nothing technically to stop this from happening, a modern flight deck can handle 99% of the workload all the pilot has to do is monitor the systems and drink their coffee.
How on earth are you going to drink coffee at FL600 single pilot, presumably with a mask on? A rapid D at that alt would spoil your day for sure. Who would get up out of the hot seats and make the coffee for you? Do you have shares in Thermos? Maybe we could have crew meals a la U2 pilots, through a straw. I agree with Pace.