I think the point is with the last radio call, yes he didn't read back the freq, yes this happens a lot, but there's normally a reason for not reading it back, perhaps you were day dreaming, filling in a log book, A nav check, systems check, shoving food down your face, chatting. The list is endless. But, what was the reason that the PNF didn't read back the frequency? What was is that lead him not too, an innocent reason? The start of something that distracted his attention? As this was the last human interaction from the flight it deserves an analysis?