IMHO this is flawed and far better to assume that one flight crew might become mentally altered at any time i.e. mid-flight ..... Allowing only one person up front is clearly a single point of failure in an environment engineered for redundancy.
But this misses the fact that even with 2 (or 5) people up front, your mentally altered person can still kill everybody.
As earlier stated, 500' on Approach, full down elevator, full power, maybe flaps up. On the Airbus, you even have the luxury of a red button to lock the other pilot's sidestick out.