Family tree are not only 6 persons. It would be kind to share my family domain with others members of my family. Today I have subscribed to a family plan for my chidren and wife but my brothers's family and sisters's family could not use the same domain name (which is our family name) with proton plan. It is the same for my parents if they subscribed to a duo plan they can't use this domain with proton. Else business plan are to expensive for individuals and families.

Because of this fact, all could not migrate to proton as main email solution.

So permit to share a domain name between family/duo plan.