I imagine a cabin between the 757/737 and 767- but retaining at six across in Y, with 18” seats, and with TWO aisles (2-2-2.) Obviously, the thinner fuselage design would only allow for either center or outboard overhead bins, but I imagine such a configuration would be quite spacious and could be, at operator discretion, converted to a terrible 7 abreast single aisle (4-3) if desired.
4-3 is not permitted. 14 CFR 25.817 prohibits more than 3 seats on either side of an aisle. This limits any passenger from having to climb over, or rescuers from having to assist across any more than two seats during an emergency evacuation. So 2-3-2 is really the only practical 7 abreast option.
Discussion about 7-abreast options generally revolves around the concepts Boeing has shown for a fuselage that is wider than it is tall. This is atypical due structural and volume optimization issues that I won't go into here. It was a point of regular contention in threads speculating what the NMA would be like.
Biofuel is certainly "cleaner" than fossil fuels. To what extent is still a question.
There have been a significant number of life cycle analyses performed on this topic of the last decade or so. These normally include not only carbon emitted due to energy use during production of the fuel, but also due to land-use effects and fertilizer production, for example. The results concur that biofuels have lower carbon emissions. How much lower varies depending on the study, the feed stock, and the production method. For example, if I remember right, simple hydrolytic conversion of natural plant oils tends to come out around a 40-50% reduction in net carbon emissions compared to petroleum, while Fischer-Tropsch conversion of waste cellulose (such as corn stocks from crops already being grown) could potentially achieve over 80% reduction in net carbon emissions.
In other words, depending which option is used, there could be a 3:1 difference in the amount carbon emitted, but all of them should still be a meaningful improvement over petroleum. So far, I'm not aware of any organization getting close to competitive pricing on the processes that achieve the highest CO2 reductions.
So Boeing announces a 737 replacement now for EIS when 2025/2030 time frame, what do you expect them to do with the MAX frames already produced, the existing orders will not be taken and the market share will take an even greater hit.
Deliver them as planned. Every new aircraft faces this problem. Unless basically everyone who has put serious effort into forecasting the post-COVID demand is very wrong about the long term outlooks, although the airlines can't afford to take many deliveries right now, later on they will need them. Airbus will only be able to ramp up so fast, and airlines won't want to wait years to meet demand.
Now if you somehow believe that Boeing will be able to invent an a/c that is 100% better than any existing Airbus product and Airbus will be unable to create a counter for the next 10 years allowing Boeing to recapture market share, that some great analysis.
Boeing's new aircraft does not have to be 100% better than what Airbus can counter with. It has be enough better, in enough metrics, to enough airlines, to be reasonably profitable.
The A320NEO is not the perfect aircraft that can never be competed with. It is a very good aircraft, though, with more potential to be unlocked, so it is critical for Boeing to define the next aircraft very judiciously. I'm certain they realized this when they put the NMA on hold.
if they are 100x cleaner than fossil fuels
Just for the sake of encouraging habitual precision, that should be 99% cleaner. 100x cleaner is mathematically impossible. Despite the fact that it has become a colloquial way to express what you're getting at, it is technically incorrect. In some contexts it can generate confusion.