The early heady days of the 787 program are over. Customers have filled the 787-9 order book often while flipping 787-8 orders into a 787-9 order. Boeing is also catering for a strong single Aisle market with the Max 737 family. Models go from 737-700 to 787-900 Max types for its order book. The 757 is being retired in a orderly progression with no heir apparent on the horizon, even though Airbus proposes a A-321 NEO. This model condition creates the often quoted "Gap" in the order book line-up. Their is no "Gap" filler from the 737 family to the 787-8. It becomes quite a jump. The "stop-gap" measure is ordering the Max 200C or the 737-900. Boeing has stopped consideration on filling its gap filling quandary in the Boeing family of aircraft, or has it?.
Data is what everyone lacks, and Boeing has this in abundance. It projects for the need of 33,000 new aircraft added to order books from now until 2030. It knows how much it will cost when wedging in another new aircraft type competing for financial resources while having both the 787-10 and the 777X family projects moving forward. It also knows several other points, the Airbus A-350-800 is an epic order failure. The door swung open for Boeing in this case. There is no competitor with the 787-8. Airbus saw this too, hence the A-330NEO is a knee jerk 787-8 stopper done on the cheap. Loyal customers responded in lock step for a few hundred orders. They weren't going to buy Boeing aircraft in the first place.
However, Boeing just sits on its ultimate plan for its family of aircraft from single aisle to duo aisle offerings. The gap is waiting for the world to catch up rather than Boeing coming out with the single aisle or duo aisle new aircraft "gap filler". They already have the gap filler in its stable and its flying today, but the world growth has not caught up. This change will only be noted in the 787-8 order book in the next few years. Boeing needs to build a 787-8 metro class variant. Featuring a 220 seat limitation, shrinking the fuel load for only for up to five thousand miles of service travel, and making weight stripping adjustments within the 787-8 frame for shorter travel legs. The 787-8 would be a perfect candidate for optimizing the use of payload limits, and making engine adjustments for better fuel consumption when flying interurban routes such as a Seattle to New York routine.
Gap pressure will come from airline growth from its customer base. Boeing must lower the purchase price of the 787 metro concept through a less is more campaign added to the 787-8 aircraft. The 737-900 has reached its route limit, and it falls just short of linking world growth and routes just beyond the 3500 mile boundary. A 787-8 Metro variant would fill in nicely with little sunk cost going into another airplane program. The Metro 787 is not made for globe trotting, but it's made for the world you and your friends live in.