Getting back to the original point: the 777 is not more efficient
primarily because it is a twin.

The cost of the 777's twin engines is probably less than the 747's
four engines, but not very much less. The larger and higher bypass
engine is a continuing step in the sustaining progression, which is
that the bigger engine delivers more total "power by the dollar-hour",
but at less unit affordability. You can't buy a fractional engine: you
either do or do not own the engine.

Small turbine engines at affordable prices-a 250 hp (flatrated to FL
200) aircraft engine for at the absolute most the price of a midsize
car-is what GA needs. A small engine in that power class could be
built for that price at existing Lycoming volumes, but that would be
disruptive development.

(Lest you believe that turbines are inherently expensive to make,
take a good look at a modern truck turbocharger. CNC machining and
investment casting have reduced costs drastically, which has not been
reflected in aircraft small turbine pricing partly because they are
old designs but partly because the market is price-insensitive. I'm
convinced P&WC could sell PT6-class engines for well under fifty
thousand dollars if they had to.)