Actually someone performed simulations of 777 left "alone", it ends with a a death spiral quite soon, it could not stay in the air for 7 or more hours.
Actually, pretty much any aeroplane, in trim and left to its own devices will stay airborne indefinitely. It will fly some very scary excursions in pitch and roll, but it will continue to fly. Go and try it.
And it also needs to be understood that training simulators don't have data to cover anything outside the certified flight envelope.