There's nothing that says that "top-down dictum" doesn't work for nation building. Indeed, looking at history, it's the only thing that ever did. From a purely bottom-up approach, your feeling of connection to someone else would be a linear function of distance to them. In such a world, it would be impossible to have a border between two people just a mile from each other, with people on each side speaking different languages.

Indeed, the whole concept of the nation-state was a top-down inventions. And possibly not the best, especially when some borders were famously drawn by British people with too much gin in their blood.