>Fuel efficiency is a major factor. Outside of countries big enough to prop up a domestic airliner industry with enough spare cash to compensate for inefficiency (which is a list of 1 country, China), nobody will buy a jet which is less efficient than an alternative.
No, it's not really a factor, because, as I wrote before: airliner manufacturers don't make jet engines. They can only get engine manufacturers to build them engines, and integrate those into their airframe designs. Of course, actually having access to modern, efficient engines is necessary to be competitive, which as you point out is problematic for Russia and China, but if, for instance, a major Japanese company decided to suddenly get into the jumbo-jet business, this shouldn't be an issue for them, provided the existing engine manufacturers have enough manufacturing capacity to provide them engines in addition to those they provide to Airbus/Boeing. So yes, being fuel-efficient is important to the end customers (passenger airline companies), but for the airframe makers, that's really determined by the engines they have access to, not really by any technical capabilities of the aircraft makers. The problem with lack of supply in large passenger jets isn't the engines (AFAIK), it's really just the Boeing/Airbus duopoly and how quickly they can build new aircraft, which use the engines from those suppliers.
> company decided to suddenly get into the jumbo-jet business, this shouldn't be an issue for them
I don't think it's that simple. You can count jumbo jet makers and their engine suppliers on one hand. It's not like you can go shop for a new line of engines on Craigslist, these things are designed with the plane maker in mind and production numbers planned for a decade+ ahead. Both parties have to work very closely together to produce something with acceptable fuel efficiency.
the major engine OEMs aren't investing enormous amounts in optimising for a startup airframe program though, so at best you're getting a derivative of the engine on the equivalent sized Boeing/Airbus program with roughly the same fuel efficiency but worse expectations in every other relevant area (expected reliability, competitive local maintenance markets, type-rated crew availability, commonality with existing fleet, residual values etc, and probably worse aerodynamics and structural weight) due to the airframe program's relative immaturity
The most fuel-efficient aircraft possible would be an electric one (for the sake of argument lets assume batteries of high enough Wh / kg would exist) flying at much higher altitudes (20km+) than the current fuel and oxygen fired engines.
That definitely involves both engine and aircraft design.
Even for jet engines: an engine from the 50's is not going to cut it in terms of fuel efficiency on ANY airframe compared to current flying machines. Even if you had unlimited budget and time to design the perfect airframe for those engines.
> (for the sake of argument lets assume batteries of high enough Wh / kg would exist)
Highly unlikely, it's not even close right now, and nothing in recent progress indicates that soon we might have batteries that are denser and lighter by a factor of 10 any time soon.
At this point the 737 Max 8 and A320 both are sitting around an 18:1 L/D. You can't really go a whole lot higher than that without affecting controllability. Things like winglets give you incremental improvements on the order of single digit percentages.
But weight savings... that's huge. For every pound you shave off, all other things equal, means the engine needs to make less thrust and the aircraft itself needs to carry less fuel.
No, it's not really a factor, because, as I wrote before: airliner manufacturers don't make jet engines. They can only get engine manufacturers to build them engines, and integrate those into their airframe designs. Of course, actually having access to modern, efficient engines is necessary to be competitive, which as you point out is problematic for Russia and China, but if, for instance, a major Japanese company decided to suddenly get into the jumbo-jet business, this shouldn't be an issue for them, provided the existing engine manufacturers have enough manufacturing capacity to provide them engines in addition to those they provide to Airbus/Boeing. So yes, being fuel-efficient is important to the end customers (passenger airline companies), but for the airframe makers, that's really determined by the engines they have access to, not really by any technical capabilities of the aircraft makers. The problem with lack of supply in large passenger jets isn't the engines (AFAIK), it's really just the Boeing/Airbus duopoly and how quickly they can build new aircraft, which use the engines from those suppliers.