Turin, it seems Boeing highly values grandfathering rights while certifying new versions of their aircraft types. E.g. the 737MAX certification is for a good part based on changes to the original 737 Type Certificate.
Point of the discussion is the enormous 777-9 will no doubt be a good machine, but hardly a 300-350 seat 777-200ER replacement. The 777-8 seems neither because of its high weight and the 787-10 will no doubt be very efficient up to 5000NM real world flights, but have weak from Asia long haul potential.
A 350 seat 8000NM capable 787 might be needed to counter more 777 operators switching to A350s
