>250 years is not a particularly impressive amount of time for a country to not fall apart.

250 years is older than almost every country in Europe (by that I mean current borders and form of government, not the ancient historical ones).

Most were monarchies or various forms of dictatorship till only a few decades ago and finally settled on their current borders only after WW2 or the fall of the USSR or the Yugoslav wars.

For example Spain had its first democratic elections in 1977 and then the UK was dealing with "The Troubles" sectarian conflict in northern Ireland. Europe always was a powder keg around forms of governance, culture, religion and sects. All that is not something that goes away overnight just because EU membership happened.

In contrast, 250 years of continuous governance and conflict free stability is super impressive by that standard.