I’m Czech, and exactly 105 years ago (October 30, 1918) the approximately dozen nationally aware Slovaks met in an inn and wrote a letter to Prague that they agree to be part of Czechoslovakia as the “Czechoslovak nation” because they knew they couldn't form a state on their own, and split off the hated Hungary. The 4 people who signed our “Declaration of Independence” 2 days prior needed someone to represent Slovakia so they went in the streets searching for a Slovak. Vavro Šrobár, a nationally Slovak lawyer who incidentally just arrived to Prague, came forth and signes the document, and became Minister of Slovakia a few weeks later.

The Republic helped Slovakia reach its industrial potential and gave its people democratic values (except for WWII, we don't talk about Slovakia in WWII). Eventually, Slovak politicians wanted power so they broke off after true democracy was restored in 1989. The Velvet Divorce was so uneventful compared to the end of Communism that people did not really care at all.

So I agree that to informed people in 1923, Slovakia being separate a century later would be no surprise. However, the formation of USSR (which I know much less about) was pretty controversial and involved a civil war so they might be actually be surprised it did last 80 years.

On the other hand, the other changes you glossed over are quite significant, especially with Germany and Poland.