I should add the guidelines and policies regarding two-crew cockpits were in place to assist in identifying the person on the other side of the door as "authorized", the opening of the door itself (this Phase I SOP was in place before automated magnetic locks and video cameras were fitted), and in cases of pilot incapacitation. The idea of a single pilot suicide was not part of the thinking at the time.
I have to admit, when we got the first reinforced doors in early 2002 they came with magnetic locks and video surveillance and thought it was the absolute norm everywhere, especially in the US. It was a retrofit and the placement of the monitor was sometimes awkward, especially on our 733s, but they allowed a clear identification from the start. And the emergency code for the magnetic locks provided a safe reentry in case of pilot incapacitation. Yes, we initially had the two crew rule but it was apparently clear we didn't need it, as door operation and identification was available from both pilot seats. And therefore there was no rule requiring a second crew as long as other means of identification and door operation were available. With either system inop (magnetic locks/video surveillance) we had to return to the two crew rule for obvious reasons. But those systems are extremely reliable, i have it experienced only once.
To be honest, i haven't seen any european airline without video surveillance, neither those i worked for nor those i jumpseated with (yes, flight deck jumpseats are still available).
The current mindset to add a second crew if one pilot has to leave the flight deck is more that it is needed as a suicide watch than as a door operator or for identification. Giving the changed working conditions for cabin crews though that is not without risks. In times of a widespread use of zero hour contracts, 6 month contracts and so on, cabin crew turnover is extremely high, up to over 50% of the workforce per season in some airlines. They only need a very perfunctory medical check before employment (can you walk? can you talk? you're hired) and none thereafter, they are not a known entity due to the short time with the company and extremely short training (4 weeks compared to 2,5 years for pilots) and have of course only a very limited background check as everything else would cost too much money. And now they are entrusted to keep the flightcrew in check and hold the lives of all on board in their hands.
As someone with malicious intent i know i would rather send my guys to a cabin crew class than trying to get in as a pilot, one is a near guarantee that they will sit in the flightdeck in a few weeks, the other is a near impossibility with failure rates of more than 90% during initial selection.