Europeans have a prediliction against ' typical US cowboy' ideas of individualistic bravado; understood. But!
Given, most flights will not have 'Sky Marshalls' assigned (just not enough to go around).
Given, 'Sky Marshall' duties as defined before 9-11, were not to prevent cockpit entry, but to prevent cabin take-over. The threat has changed; it is now sudden attack against the cockpit. To prevent such attack, 'Sky Marshals' must act as a permanent barricade against such sudden storming of the cockpit. They can't be unknown pax, spread through-out the cabin, because their reaction time would be too slow. To work effectively, there has to be a physical, gun-proof barrier behind which they would stay, guns at the ready; anything else would not signify. I am talking about a WWI gun-nest, with one-way mirrors & bullet-proof walls, between pax & cockpit. Without this, each time a pilot takes a pee, or the F/A takes food/coffee/gossip forward, there is unacceptable threat of takeover; the idiotic 'strengthening' of cockpit doors is laughable. We all know such a 'barrier' strategy will never be put in place.
Given, 1)most flights will not have marshalls, 2)even with marshalls, their deployment will not be effective, and 3)the cockpit door is no barrier at all, an armed cockpit crew is probably the only reasonable answer, inadequate as it may be.