For all that suggest having a CC member on the Flt Deck is nonsense or otherwise unnecessary - would you have thought the same had you been the Captain on this flight? Does that matter?
It matters, but only when added to the safety case / experience to date - not as here, taken in isolation.
Others have quoted 9/11, other suicides / scenarios and the risks this policy brings in. It's not even certain the policy would have stopped this, just altered the exact modus operandi.