So your thesis is ~100 years ago was some freak historical period when countries were mostly homogeneous (compared to current day. Please don't cite e.g. Czechs and Slovaks living in the same country as diversity comparable to current day), but throughout the rest of history, diversity was much greater.
Do you have sources to back this up? And how did those countries become homogeneous, then?
How do you think all those European Americans got there?
And, modern nation states are very new. For example, France drove to extinction or near extinction all languages besides French spoken in what is now present day France through government policy. Like the language of Bretagne.
Your profound misunderstanding of history is not something I can rectify with citations. Read a book? Like, a scholarly book.
