The main issue for building a narrowbody is production rate. They have to fly off the assembly line in huge numbers. Right now that means the 737 and A320 are here to stay for at least the next 50 years. However, Boeing is going to be at such a competitive disadvantage in 10 years with the 73, that they are better off exiting the narrowbody business, and only building widebodies, while letting Embraer deal with less than 200 seat aircraft.