Could it not be possible that Boeing are considering long-term the 737 replacement and therefore, will pitch a single aisle that covers the 757-200/300 initially (220-270 pax) with an all new fuselage, wing and engine. Then, using this fuselage ultimately an integral part of the 737 replacement?
If there is talk of making a twin aisle with single aisle economics surely airlines would consider a single aisle with even better economics?
As engine efficiency improves and materials become lighter/stronger it has a flow on effect on the size if the plane.
The 737 replacement that seats the same number of passengers as a 737-8 will then be much lighter. It will require a smaller wing, less fuel capacity and engines with less thrust to perform the same mission.
It would be impossible to make one wing and engine combination that could scale from the 737-8 market up to the 757-300 MOM market. This would end up with the 737-8 size being overweight and the 757-300 being underwinged with poor lift to drag. So both models would get beaten by an optimised design.
The A321LR is already shows signs of this. It is clearly underwinged and the A320 is overweight compared to the 737 on short sectors.
The MOM 797 needs to be a carbon shortish 8AB to get that 757-300/767-200 capacity/range target. It will mature carbon fibre production to get costs down.
The 737 replacement will need to be a carbon 6ab, 200 seats and fully optimised for maximum efficiency under 3000nm. The CS500 would provide the minimum benchmark. The CS500 would be around 10T lighter than a similar capacity and range 737MAX. No oval 7ab fuselage here.
A tight, 6AB, fully optimised, carbon, short ranged aircraft from Boeing could easily trump the CS500 which is too wide for only 5abreast.