More than one of everything is needed for redundancy. From airliner engines to hydraulic systems to pilots. Going from three to two pilots might be okay somehow but below two is not enough anymore.
Using a cockpit environment made for two pilots with only one pilot in the room is just dangerous. If a single pilot environment is wanted for some reason the entire cockpit must be made for it and certified from the beginning. I can't imagine to see that happen in an airliner. Realistic failure scenarios have to include electrical failures, physical damage and fires with no automation and no datalinks left. This is when you need more than one pilot. In the cockpit not just on the plane. We have seen with the first lion MAX event and with Qantas A380 how much better some enhanced crew could handle unexpected events.
Saving costs by just cutting layers of redundancy would be a quite a paradigm change in aviation.