There have to be two types of admin users:
1. Super admin with all privelegies
2. Admin user

The super admin can see and edit all entities.

The admin user can see/use all public entites (for example categories) where the userId is NULL. He can create private entities, only visible for himself or super admins, the userId is the current admin user.

Why this idea makes sense?
Technically we just add a new context variable like the salesChannelId or languageId, the admin user can manage his own product sets without any risk to delete other products.
We could also make a step forward and add userGroupId as the owner, so a selected group of admins can access their own data.
This would also give a good base to make Shopware 6 ready to use as a marketplace system.