w1pf :
Yup, no free lunch. But we don't need a free lunch -- we don't even need a really cheap lunch. What we do need, if we're going to give an automated system authority over the most powerful control surface on an aircraft, is a system design and implementation that takes every reasonable and practicable step to make sure that the inputs upon which it is relying are valid.
Whatever the cost of that insurance -- sensor fusion, voting logic, simple crew warnings, etc. -- is virtually certain to be less than the cost of not providing it and letting a confused HAL fly airplanes into the ground. What could be more expensive than killing 150-200 souls, and losing a $120M hull, because of a sensor failure and an unmanageable sensory overload on a flight crew?