I can’t go all the way on that one, because there are some very loaded legal and technical terms in there that go out on a limb (catastrophic, requiring, active, unstable, normal) that I do not know as fact.
Clearly, though, there are reasons why they did not require dual channel inputs, did not have a fail-safe system, why it was completely hidden from pilots (both in the OFFICIAL, LEGAL AFM and on flight deck displays) and they ALL point to not making the aircraft APPEAR TOO DIFFERENT, so as to APPARENTLY avoid the expenses you correctly assert.
This reminds me very much of the Thales pitot-static system issue on the Airbus, but that was a known problem. Single inputs to the aircraft’s main flight data computers can make serious problems out of simple ones, because they are used in so many ways by the computers to make the aircraft “easier to fly.”
And, yes, Boeing and the FAA appear set for a circular firing squad in court. Boeing will say the FAA approved EVERYTHING, while the FAA will say Boeing DIDN’T TELL THEM everything.
These accidents could well make aircraft certification considerably more expensive and time-consuming in the future.
The more near-term expense is one of reputation, with potential lawsuits being eye-wateringly expensive, too. In the 1999 crash of a DC-10, the average payout to families who lost a loved one was about $4M, and that was for an accident that was a result of an invisible hairline fracture of a single turbine blade that cut through the only place where all 3, supposedly redundant, hydraulic systems met in the back of the aircraft. It was a million+ to one circumstance, at best, and could barely even be imagined. One assumes that the payouts for this kind of accident 20 years later, where there is already huge outcry over the short cuts taken, will be in the region of TEN FIGURES, if not 11!
I expect the lawsuits to go on for the rest of my 9.5 year airline career, and possibly beyond!