A lot of people who call themselves "citizens" have no loyalty, either.
Cultural and genetic heritage exists, but its boundaries are in a mutual feedback relationship with political borders. Culture and genetics might cause political borders to be where they are, but they are also caused and reinforced by where political borders happen to be. (The law makes it difficult for you to travel to America => You're unlikely to be immersed in American culture or marry an American => Your culture and genetics remain very distinct from American culture and genetics => People see no need to facilitate travel to America => Repeat cycle.)
So if borders and border control policies changed, the boundaries of cultural and genetic heritage will follow suit, and vice versa. This means you can't just appeal to existing cultural boundaries to argue for some sort of nationalism that perpetuates said boundaries. So although I agree with you that borders aren't completely "imaginary", they're not completely real and static, either.
Cultural and genetic heritage exists, but its boundaries are in a mutual feedback relationship with political borders. Culture and genetics might cause political borders to be where they are, but they are also caused and reinforced by where political borders happen to be. (The law makes it difficult for you to travel to America => You're unlikely to be immersed in American culture or marry an American => Your culture and genetics remain very distinct from American culture and genetics => People see no need to facilitate travel to America => Repeat cycle.)
So if borders and border control policies changed, the boundaries of cultural and genetic heritage will follow suit, and vice versa. This means you can't just appeal to existing cultural boundaries to argue for some sort of nationalism that perpetuates said boundaries. So although I agree with you that borders aren't completely "imaginary", they're not completely real and static, either.