The political and market forces which will dictate the next airplane programs at Boeing and Airbus remind me of the political and economic environments in the late 1970s. In the late 1970s, we were seeing high oil prices and concerns about oil supply. The government response was renewed research into programs which would reduce the weight and improve the performance of commercial aircraft. I am most familiar with the NASA ACEE (Aircraft Energy Efficiency) program, which has been called the "Apollo of Aeronautics" (https://upload.wikimedia.org/wikipedia/commons/0/04/The_Apollo_of_Aeronautics.pdf
In the aerostructures discipline, the NASA ACEE program funded prototype carbon-fiber primary structure and control surfaces--L-1011 inboard aileron, DC-10 upper aft rudder, 727 spoilers and elevator, and later a 737-200 horizontal stabilizer at Boeing (https://ntrs.nasa.gov/citations/19950022068
). The importance of these programs is that by constructing prototypes and placing them into service it gave both the OEMs and the regulators the knowledge to proceed with full production composite components, the 767 being the first with CFRP control surfaces and later the A320 with its horizontal (HTP) and vertical (VTP) stabilizers.
I note additional research at NASA under the ACEE program looked at engine diagnostics, which is the predecessor for the current engine health monitoring (https://ntrs.nasa.gov/api/citations/19780019105/downloads/19780019105.pdf
Although separated by only a few years, the 757 was developed at a time to better take advantage of this research as it has a much better airfoil than the 767. The 7J7 airplane which was begun to fulfill a Delta RFP for a 150 seat airplane would have been the logical extension should oil prices remained high. (Saudi Arabia massively increased oil production in the mid-1980s which collapsed oil prices, and which in turn precipitated collapse of the USSR, but I digress.)
The European Union has a much better pulse on these trends as it is funding a host of aeronautical research and development programs.
Just yesterday, Airbus announced that it will be relying upon Sustainable Aviation Fuels to meet its 2050 carbon pollution goals. This is not a low cost approach. We can easily foresee how fuel efficiency is going to be the predominate design driver for any new Boeing or Airbus product.
So with that background, what is the 737 replacement? The problem here is that the 737 is currently used as a very versatile platform with some airlines flying it under very short stage lengths, like Southwest, and others using it at its maximum range of transcontinental and even transatlantic, like Norwegian. It is my premise that under what will be essentially high fuel prices that one platform cannot sufficiently replace it. The 737's versatility comes at the price of fuel efficiency.
A wing optimized for short stage lengths will be a different wing than that optimized for longer flights. A short range 737 simply cannot be affordable carrying along the weight that is imposed by the design that a long range 737 requires. Consequently, the Bombardier/Airbus A220 will be supplanting 737s on the low end because it's economics (read fuel efficiency) is superior.
So I'd postulate the 737 replacement needs to be two separate airplanes with a common fuselage, common flight deck, common-type rating and common systems--one a short-range optimized airplane for high cycle and short turn times which would fit within current airport gate limits and a second airplane which would be unconstrained by gate limits which would be optimized for US transcontinental flights and flights lengths up to where a single flight crew may be used. A folding wing, while technically feasible, will not buy its way onto the long-range airplane because of the weight and cost and the airports able to accommodate a mixed-fleet of short and long range airplanes. It is a different situation than 777-9 where the folding wingtip was required to meet the taxiway separation requirements.
Certainly with the quantities of airplanes that both Boeing and Airbus are manufacturing at upwards of 70 per month, one can see how the business case could close for such a two-wing approach.