Not everyone in the guild able to access shadow pokemon area even guild in pve ladder, similar with pvp not everyone able to access guild island because requirement. It is fair to everyone. Also, that area only 1 hour able to go inside not like guild island access 24/7.
In addition, dungeon still need Twin Dragon Orb to access Victini dungeon and only obtainable in Medusa/Eldir and Lance boss, it is not easy to get Twin Dragon Orb and still need to wait 2 week for a battle boss. Unlike PvP just stay Pokemon Center and click the Pvp button.
Therefore, I am not agree let everyone to access it.