Here's an astonishing fact: The Air Force recently announced that the B-52H, of which 74 are still in service, will remain in the active fleet until 2045. By then, the design will be 93 years old. Lots of technologies—steam propulsion, the internal combustion engine, stick-built houses, the air brake—have lasted longer than that, but I can't think of a single airframe that betters that. The DC-3 is close, but how likely is to be in meaningful commercial service in 2028, on its 93rd?
Push the playhead forward into the imaginary future and focus it on aircraft subsystems, especially primary controls. Then ask yourself if 90 years from now, primary controls will still be mostly direct cables or if, as Diamond Aircraft now seems to suggest, they'll be fly by wire. It's not quite a chicken-and-egg question, but more like if it ain't broke, why fix it? (The B-52H is loaded with cutting edge avionics, but it still has the same control system it left the factory with a half century ago.)
Technologies persist for several reasons. One is performance. If something works well and its replacement does no better, inertia rules and it endures for decades. Cost counts, too. Some electronic systems are cheaper than the mechanical ones they replace or cheaper than the previous generation of electronics and have greater capability than what went before. That's a split decision for aircraft avionics; for upgrades, they're generally more expensive as a percentage of airframe value than legacy systems, but they definitely do a lot more. It's debatable if "a lot more" means you can fly in weather you couldn't before, the airplane goes further and faster and/or is just safer. Essentially, modern avionics mean you have better situational awareness because you have more information. So far, we have aptly demonstrated that this hasn't moved the accident rate downward. In avionics, we've certainly proved an affinity for bright shiny gadgets.
In aviation, at least general aviation, markets and makers tend toward the conservative. The market has not rewarded innovative airframers or engine designs. (Before you say what about Cirrus, I'd say it's incremental technology, not revolutionary and I'd raise you one Beech Starship in the process.)
Consider the new Rotax 912 iS, a nice little incremental improvement on the 912. But it's hardly a technological tour de force. It doesn't yet have direct injection and it took us all the way to 2012 to get rid of carburetors. These decisions by Rotax were made for sound technical and economic reasons, but they're driven by buyers whose idea of a walk on the wild side is a Cherry Coke.
One reason some perfectly adequate technologies don't persist is that they're overwhelmed by the macro economics of developments in related or parallel fields. In case you haven't noticed, electrical controls and motors of all sorts are replacing mechanical controls in cars. Throttle by wire is already common on motorcycles and cars. Next year, Nissan will offer steering by wire on the Infiniti. Goodbye tie rods, steering racks and power steering pumps. Dynamic, electronically controlled suspension systems are commercially viable. On the drawing board are electric brakes—even for motorcycles—and electric valve actuators for cars, eliminating the camshaft and valve train entirely. These aren't widely economically established yet, but they aren't just wild-eyed concepts, either. Twenty years from now, the world might be awash in cheap digital actuators and servos, just as it is now awash in cheap MEMS gyros made possible by huge volume in automotive braking, sensing and navigation systems. That's why Dynon can make a $1500 battery-operated EFIS.
And to me, that's the big unknown about Diamond's fly-by-wire project. On the one hand, cable-controlled primary surfaces work well, they're light and cheap. With cable or rod-controlled surfaces, you feel the air. Feedback is part of the tactile experience of flying. But is an important part? That steer-by-wire Infiniti will numb the driver to how the tires feel when they're about rattling down a washboard road or perhaps at the slippery edge of hydroplaning.
So fly by wire may not be about feel at all. At Diamond, CEO Christian Dries hopes it will bring along features like autoland and higher performance airframes which can't be achieved without what will eventually become automatic flight control systems. It seems so tantalizing and so obvious, but given the industry's preference for sticking with what works, I don't think anyone has a clue which way it will go. It will pit aviation's hidebound conservatism against harsh economic reality.
I suspect it's rather like Boeing turning out the last of the B-52H's in 1962. They could not have then imagined that those old control wheels would eventually be horsed around by pilots whose great grandfathers laid hands on the very same airplanes. Nor could they have foreseen the old warhorses upgrading through four or five generations of avionics that might, for all we know, result in B-52s flown to the boneyard via fly by wire. Not sure I'd bet against it.