Why have a cockpit door at all? Why not turn it into a bulkhead? It wouldn't be that hard to do. The flight crew would gain access through either door R1 or L1 (ok, so extra airstairs would be req'd). The fwd lav is already there, so there is no need to move it, and add a small galley.
To modify current aircraft would be quite costly for the operators, so I agree that beefing up the existing door is an adequate solution for now. But there is no reason why this couldn't be incorporated to new aircraft during manufacture.
The fact of the matter is that as long as there is a door that connects the cabin to the flightdeck (no matter how well you secure it from the inside) there is a possibilty that someone could interfere with the pilots. Terrorists could keep thinking of ways to get through that door.
If there was no possibility to gain access to cockpit in flight, then hi-jacking would simply be a thing of the past.