Earlier quoted context omitted.
None of the engine manufacturers are interested. That's what killed the A380--Rolls Royce pulled out of a re-engine deal that would've brought A380 engines into the 21st century. "Green" concept showcases notwithstanding, all the engine manufactures seem to have decided to cut back on investment and coast for awhile. Something will need to change to spur competition in the engine space, and then we might see hungrier…
The latest and greatest engines are not that far from the theoretical thermodynamic efficiency limit. So future designs are running into the hard part of the asymptote. I doubt airlines are actually interested in committing tens of billions of capital, today, to get a dozen % improvement in fuel efficiency a decade from now. And why would a private investor take such a huge risk without guarantees of making it back s…
Putting my cynical cap on, my hot take is this: Everybody now realizes that Emirates and Clark are exactly right. As was Airbus.[1] Demand will blow past supply (i.e. runways and gates). Ticket prices will sky rocket. But from the perspective of the plane and engine manufacturers this translates to, "if everybody sits on their hands and does nothing, we're going to enjoy a windfall of profits". Clearly, IMO, Boeing, all the engine manufacturers, and now (somewhat despite themselves) Airbus, are going to passively collude[2] to bring about this windfall. All they need to do is sit tight and wait things out. The threat of competition isn't significant enough to break the collusion; there are too few players, the barriers to entry--regulation, technical and logistical expertise, etc--too steep, and like with housing little prospect that political reforms will make it easier to build enough new runways and airports to change the capacity trajectory.
Similar calculus applies to most of the established carriers. There's much more competition among them, but if the manufacturers are expected to continue to coast there's no angle to exploit in that regard and thus little incentive to agitate for disruption along this dimension, and so the carriers also get to look forward to windfall profits, notwithstanding that they'll be forking over much of it to the manufacturers. Clark just isn't cold-blooded enough to want to play that game; he's got too much of a fighting spirit. (Or perhaps more realistically it's also because if Emirates doesn't keep growing it won't be viable in this prospective future reality?)
[1] Boeing wasn't wrong, either, but point-to-point only bought a couple of decades of supply expansion.
[2] I think there's an economics term for this, but I don't remember it.