Saying this is essentially dismissing someone else trying to correct the ignorance that Estonia, Latvia, etc. are the same or have similar cultures. This isn't highlighting differences any more that someone correcting someone else that USA and Mexico are not the same countries because they were both colonialozed by europoean countries.
Where I am people have a centuries past idea that our neighbouring regions are our enemy, we're so different, look hire different it cultures are. Having traveled more widely than is the norm here (an area of the UK) it seems very similar and the supposed differences in culture look like similarities.
It's like people tell me my kids are very similar in appearance, to me they seem very different. The differences in human culture in some areas are like "well they a clog dance wearing bells but we do it with ribbons, see how different we are".
We seem often to look for the differences and use them as a way to "other" people, that is to separate from them. I think we should be guarded towards that, particularly when the differences are things politicians have decided for us to emphasise and express.
I'll bet there are many similarities between people either side of USA borders. However, as you say you are then looking at long-term indigenous populations compared to more recent immigrants (European settlers). The cultural aspects influenced by conquistadors and those from French and British culture will bear some similarity, I'd expect.
Well, Latvia and Lithuania are not Slavic in culture, save the culture pushed on to them via Russian domination. Their languages are related to each other, but are completely different to the Russian (and other Slavic languages) being in a different branch of Indo-European, with almost the same relationship as say, French and Dutch have with each other. Their folk culture has similar roots, but then so does the culture of say, Romania - another Eastern country in Europe that is not Slavic. I know a bunch of Latvians, and they are very different to the Poles, Bulgarians, Czechs and Russians I know, both ethnically and in attitude to life in general.
To be honest, even lumping the Slavic cultures together isn't really satisfactory, because even the Czech/Slovaks I know are culturally quite different to the Poles, and they are from a similar region in Europe and speak languages that are relatively close to each other.
Estonia is even less related. Their language is in a different language family, same as Finnish, and their culture is again very different to Russian.
I'd be uncomfortable with erasing the entire cultures and identities of millions of people against their will and forcing them to only care about boring mammal-level similarities with strangers.
I love diversity. The idea of forcibly eliminating it like that gives me a terrible feeling.
"Humanity needs to focus on what unifies us rather than what separates us."
Sadly, humanity is not a one, big, happy family and you can't dismiss hundreds or thousands years of history to create one. Globalization is the first step towards unified world, but as we can see now, it doesn't work so well. It just doesn't happen overnight. The first thing to a better world should be accepting difference and stop trying to force our values on other people. Invading and occupying neighboring countries does not help to achieve that and divide world even more.