Compare the accident rate of single pilot multi engine aircraft operators and that of airlines and you will have your answer
Pilots are more than capable of flying any modern jet single pilot. But to do it to required safety standards necessitates 2 for a multitude of reasons. Such as:
Redundancy
Gaining experience before being put in command
Stopping guys doing stupid stuff
Interaction with cabin crew, pax, atc
Etc etc etc
Single pilot cruise is the very maximum I can see happening anytime soon.
Even that I am heavily sceptical