Practically, to verify the credentials of all who would wish to join the forum would be a huge task. Far simpler, less cumbersome and fairer too would be simply to make the moderators screen the posts. Any dumb or diversionary posts can simply be killed at source. Some immediacy might be lost - but that shouldn't matter: Pprune is not CNN - but if all posts were screened by the moderators before going up on the board, everyone would be happy. No silly or ill-informed posts on the site, and no need to have special membership conditions or fees.
I offer this as a practical solution because serious discussion of safety issues should involve ATCOs, LAEs, meteorologists, communications specialists, systems safety engineers, air accident investigators, retired air crew, cabin crew, people involved in aircraft manufacture, software specialists, materials specialists -- maybe even a manager could have something useful to say. I would go further and say that it is an indication of a less than ideal attitude towards safety to assume that only pilots have anything worthwhile to say on the subject. Many of the most lively and informative threads Pprune has been responsible have had central contributions from non-pilots.