Ironically, those same twin engined efficiencies are hitting a brick wall. As the engines got bigger and bigger the fuel efficiencies went up but so did the drag. What is the 777X fan diameter now? 110 inches or something? They are also losing clearance below the cowling. They can either jack up the gear even higher, which poses so many problems, or make planes with high or mid wings instead of low.
Or maybe place the engines overwing? That makes maintenance harder.
High or mid wing means the wing doesn’t block as much of the noise in the cabin like it does now. Engines at the tail present balance problems if they’re that big.
Distributed propulsion may be the next step. A pair of powerplants, located wherever, or mixed power and batteries (or even batteries only or fuel cells who knows?), electrically driving numerous fans/props/ducts/whatever all over.
To be fair, I don’t blame the engineers for 380’s failure. It was perfectly capable at an engineering level, its numerous foibles over the years being par for the course for any big plane. It was designed to the specs the execs came up with and the execs misread the market. Sure that’s fortune telling to some extent, but that’s why they pay them those inflated salaries (like any exec does 100x the work of some line worker), to be right when it counts.