It’s true that the old Polish system of government, requiring unanimity from an assembly of nobles, seems like a bad way to run a country. But I think the theory that this bad procedural rule was responsible for Poland’s elimination as a country isn’t all that well-supported by the evidence. Poland re-emerged as an independent country after World War I, but it found itself once again partitioned by Germany and Russia in 1939. Then after WWII it was formally independent, but in fact under Russian control.
Long story short, system of government aside, Poland is located in an unfavorably geographical position that makes conquest by Russia and/or Germany a very likely outcome. It’s a great triumph of that liberal international institution-building and norm-building has now created a situation where everyone can be very confident that Poland can stay independent. That’s a big win for Harry Truman and Jean Monnet and Helmut Kohl. But given the international situation that existed 200+ years ago, I think Poland was doomed one way or another.