TripleBravo: The unit cost argument you make leaves a bit to be desired. Those ADIRUs are so expensive in large part because they will be built in quantities over years that Toyota builds a single model in a single plant in a day.
The benefits available via FBW in autos or GA simply haven't justified the disadvantages and economics to this point. FBW made sense very early for large aircraft because of the tremendous advantages it gave designers. You'll note that FADEC is in all recent automobiles, and throttle-by-wire is increasingly popular. Certain luxury cars are nearly 100% brake-by-wire right now, and most cars have ABS and EBD.
Pure mechanical controls will be around in GA long after automobiles shun them, much like antique environmentally indefensible engines burning antique fuel still predominate in GA, despite being outlawed decades ago for new autos in civilized countries.
It is annoying how certain luddites refuse to recognize that good design is good design, and to the end user it really doesn't matter much whether it is mechanical or electronic if someone screwed up and it goes badly. There are many ways in which a simple hydraulic steering boost can go badly, and I've had one lock up on me before. That recently mentioned MD80 accident was all about failing to lubricate a pretty simple, pretty life critical component. Simple doesn't equal safe, and complex doesn't equal dangerous.
I certainly hope the DFDR is healthy enough, and I hope the authorities are successful in swiftly determining what went tragically wrong here.