Limiting frontal area=drag is a design goal. Wide fuselages create a lot of unused frontal area on the cargo deck as well as above the ceiling, like on the 777.
Indeed. Which is why you should either a) shrink the width till you have a rather limited attic, as on A300/A330 family, or b) expand the fuselage until you have sufficient headroom on upper deck. 777 is too wide for a single-decker and too narrow for double-decker.
uses the lower part of the 777 standarizing cargo hold, wing torsion box etc
But you are violating the circular shape of the upper fuselage and losing all the advantages of a pure tension bubble structure. This costs immensely in pressure vessel weight and therefore induced drag.
As you can see the upperdeck of the ecoliner has a diffrent shape then the 747 upperdeck, making room for serious luggage stowage and use as 6 abreast economy a serious alternative, unlike the 747.
Ah. So 6 abreast economy on B747 is not serious, for some reason?
If you want a plane with upper deck slightly wider than 747, but not as wide as that of A380, then you should make the main deck wider, not narrower, than that of B747. So, what I suggest is a slightly expanded 747.
A ten abreast maindeck is not required for the envisaged market segment, below the A380, 370-500 seats. It can be done like on the 777, but is not standard for mainline long haul services.
If you stick to 9 abreast main deck then you cannot make your plane bigger than 777-300, basically. There is a big gap between 10 seats in the whole cross-section, as on 777, and 16 seats, as in 10 on main deck and 6 on upper deck for 747. But 15 abreast, 9 abreast main deck and 6 abreast upper deck, is IMO so bad for the pressure vessel efficiency that you are better off accepting 16 abreast and shrinking the fuselage to 747SP length.