Random events always happen. Every country will always experience a random bad leader. It will happen sooner or later. What is best then to have a land area unified into one or split into 10 different countries? One bad leader among 10 occasionally will not pull the whole area down. One bad leader for the whole area can pull the whole area into the dark ages.
Consider the Chinese emperor which banned all Chinese naval exploration and trade? That could never happen in contemporary Europe, because if your country got such a dumb leader your neighbors would quickly outcompete you and it would become very obvious how stupid that decision was within short time.
Being political divided is thus a good insurance against doom. Imagine someone like Hitler rising to power in a unified Europe? There would be no way to counter his madness.
That is why I think that China's rise today will eventually end up being a temporary thing. It will eventually stagnate, because China doesn't have anything to clearly challenge it. I also think this will be America's downfall in the end. We can already see an almost Chinese way of seeing the world in America. Instead of Mandate of Heaven, we got American exceptionalism, which means America is very poor at absorbing ideas from abroad. The thinking is always that America is best and has nothing to learn from the foreigners. Not all that different from the Chinese view when the British came knocking on their door.
What is great about Europe is that we got this big playground for political and society experiments. Different countries try out widely different models for society. I see this among Nordic countries as a microcosmos. Each country try out different ideas and then we copy what works from each other. Many of the policies e.g. enacted in my home country Norway has previously been tried out in Denmark or Sweden.