Conf, I am not sure I agree with you on the all or nothing point you are making.
Let me compare what Airbus does with the cascading control laws when various inputs go wrong to what Sikorsky does with the levels of flight control automation for a Seahawk helicopter. (Note: Not FBW in a general sense).
When all of the buttons are green, you have AFCS which includes the trim, SAS and boost features. You can disable AFCS (which allows for a variety of "hold" autopilot features, like altitude hold, airspeed hold, heading hold, etcetera) and keep the SAS, boost, and trim. You can also slowly but surely disable or lose those servos and their features as well, and end up with a "boost off" flying mode. This is a challenging but flyable mode. You are your own SAS system, and your touch on the stick (cyclic) has to be very light, even though your feet (rudders) and left hand are working very hard. (Aside: "boost off" is a slight misnomer in the Seahawk, since there is hydraulic power still driving the gross inputs into the rotor system). The inputs into the flight controls that feed the rotor system controls have a boost piston that can be diabled if there are problems.
Given a choice, I would prefer not to go from "all systems on" to "boost off" in instrument condiditions where a chance of keeping a few features, depending upon failure mode, could be worked in.
Granted, not the same kind of aircraft.