I think that noone serious proposes to include rules like > You could have rules that say you can’t link Finland to Barbados. in the data model. That is a red

I think that noone serious proposes to include rules like > You could have rules that say you can’t link Finland to Barbados. in the data model. That is a red herring. But rules like "area must be a valid area" are a good idea, in the same way as Wikipedia is requiring article code to be a text and is not allowing saving binary data there.