Can we completly discount the benifit of the deterrent effect on an unstable mind of a second person in the cockpit?
No, we can't. In this singular case. The next case will be different.
So, it is essential that sensible decisions are made but the professionals will not be there to help that decision making process if they abstain from it because they don't realize how far reaching it could be and that they cannot stop that decision process happening that will affect everyone in aviation especially front and rear crew.
I do agree. And yes, of course the professionals will be in that process, or at least try to be heard. Interestingly enough the relevant minister in germany is already talking about removing that dreaded door altogether. That of course will never be possible in the US and probably the UK neither. As always, the decision process will take a lot longer than the limelight of the public will on it and there is a chance that cooler heads will prevail once the politicians and the press have something juicier to talk about.