Average line crews have repeatedly proved they are incapable of reliably manually monitoring airspeed when their normal operating mode is ATS or A/T
In my experience, they are capable. They just don't believe it is necessary with modern technology and are too blase to be bothered.
Do you honestly think that none of these qualified 4 pilots observed a speed decay or decided not to speak up ?
Yes, as a matter of fact I believe that it is entirely possible! Whether or not this was the case in this accident remains to be seen.