The doors have since inception constituted an opportunity for a sole person to do what he wants with the aircraft without interference.
As long as the problem of pilot suicide only manifested itself in third world airlines, we first world folks perceived ourselves safe. I still remain disappointed that the regulatory authorities took no action the first time this happened.
Human nature is such that where there is an operational weakness, somebody someday will take advantage of it.
Back in the days when Stevenson et famille were putting up lighthouses in remote locations, much thought was given to minimum crewing.
Two keepers were deemed insufficient since one keeper could quietly do in the other and claim an unfortunate accident without any witnesses to the contrary.
The lighthouse minimum crew was set at three.
As for cabin crew being up front, many are given the training to help out in case of incapacitation. What we have here is the most severe form.
As for limited hours flown, military pilots are flying fast jets with far fewer hours.