ATC Watcher, you appear to overlook a key human factors point that no human can be expected to have ‘perfect’ alertness. There will always be errors; the safety objective is to reduce the frequency of error occurrence, or the severity of their effect if not detected.
Error detection, even with dual crews cannot be relied on, thus there is a need for ‘systems’.
I agreed that the operating human must not rely on these ‘error detectors’, and when they fail there must be a clear indication of their unavailability, e.g EGPWS.
I don’t think that the human aspects can be discussed in isolation. Safety defences involving human behaviour can only complement a range of other ‘error’ defences.
Also, it is not necessarily the number of defences that there are (defend in depth), as the quality of the existing defences. Many posts have identified many excellent defences in existing operations.
The current range of defences involves systems, management aspects such as procedures and safety culture, and at high level, input from the regulators.
In the latter instance the regulators could research contributing factors in this accident, as above (#2525), or mandate a new warning system. An earlier post identified
AMC 25.703 Takeoff Configuration Warning Systems (Page 368) which discussed the philosophy of TOCWS including the latest ‘high reliability’ systems already - voluntarily, in some aircraft; a simple retrospective mandate would improve this aspect.