cargun - you have valid points on system design, but you have been misled by all the 'spam' on here.
Now Boeing claims that in case of malfunction of RA 1, FCC would consult RA 2.
A/P B does not 'look at' RA1 AS FAR AS THE PILOTS' TECH INFO IS CONCERNED. Also, as far as anyone knows, there is no feature for an automatic 'swap' of RA in the event of failure of one. The troops are out on that one and we may all learn something soon.
RA1 feeds AP 'A' AND the Autothrottle, and other things, but not A/P 'B'. If RA1 FAILS, you lose the a/throttle input and GPWS warnings and an input to the rudder control function - that is all. A/P 'B' stays fully serviceable and useable.
NOW, TALKING SINGLE CHANNEL APPROACH, A/P 'B': (as we understand at AMS)
The sole function of RA2 is to feed A/P 'B' with radio height to reduce the control inputs the A/P will make to track the landing beam as the a/c gets nearer to the runway. A/P 'B' takes
no notice of RA1.
The sole function of RA1 is to feed info to the A/Throttle and provide GPWS info (and rudder). It is quite acceptable to operate with either u/s as long as the crew are aware of certain limitations, and in fact both u/s, while not 'allowable', would not constitute a major problem.
As to your overall points - I'm sure Boeing will be reviewing this now.
foresight - you are confusing cargun now -
The only time the radalt is of vital importance is during a dual autopilot approach and autoland.
- not true. Loss of a RAlt renders that A/P approach mode u/s - read the MEL!