I'm trying to think of KISS aircraft attitude instruments that would not be affected by electrical (hardware or software) failures, physical properties of aircraft ambient air, initial calibration concerns and attention distraction by synthesized sounds. Spirit suspended turn-and-bank indicators and magnetic compasses come to mind. Pitch and roll only dependent on gravity and yaw only dependent on the earth's magnetic field. Unfortunately you can't patent any of that so there's no money to be made pursuing development and lawyers still can't find a loophole in the law of gravity to bill for.