2 in the cabin seems to me to be a reasonable way to approach a very low probability orf problems scenario with a relatively low cost. You're probably talking losing a duty-free sale every few flights because a FA was called to the cockpit, and while i know it all adds up in a large company and all, still far more cost-effective than making changes to the planes themselves.

You can't make much of anything 100% risk-free, but 2 per cabin does seem to take an already very small risk and make it even smaller. It's not going to be easy disabling a second person in the cockpit (even the 85 pounds soaking wet petite FAs found on many Asian carriers can be very physically tough and capable as the Asiana crash showed) in a way that is both silent to passengers and cabin crew just on the other side of the door, and quick enough that the bad actor can incapacitate the other and then go back and make sure the door code stays overridden so the locked-out aviator can get back in.