It is the nature of the issue that there is no perfect solution; either someone is left alone and can act without intervention, or a 3rd party has to be allowed into the cockpit. Procedure has to be decided on balance of probability, and statistically a pilot alone is higher risk.
It is reasonable that some point out the new risks introduced, but this is not sufficient to render the policy ineffective, if the risks it mitigates are greater. Cabin crew already come into the cockpit in any case, so if they are a threat then that is present regardless. Reasonable concern is fair enough, the problem is I have read some pilots who are merely indignant at the idea their mighty self should possibly be supervised by lowly cabin crew, and do not see the bigger picture. That is not constructive. You are not being judged, in how many industries would someone in charge of such a potenially destructive missile be allowed to lock themselves away? I venture none.