Stavrianos' History of the Balkans Since 1450 paints a different picture than the one you give as to constant fighting in the region.

There is only one area that saw a lot of fighting, and that is in the Krajina in Croatia. That's also the area between the Austro-Hungarian Empire and the Ottoman Empire. Serbs were actually recruited to that area as a bulwark against the Ottomans. But the Bulgars, Albanians and Serbs were firmly entrenched inside the Empire, and for hundreds of years, there was no fighting. The 19th century saw the first independence movements in the region, and there was a great deal of fighting then, but it was against the Ottoman Empire.

I would subscribe to your point of view for the 20th century, especially the first two decades.

Afterward, the Balkans were swept up in Axis-Ally intrigues with Albania, Bosnia, Croatia, Bulgaria, lining up with the Nazis and the Serbs and Greeks lining up with the Allies.

After the war, nothing happened until the 1990s.

As to the geostrategic importance of the region, it used to related solely to the Danube, and the fact that this river was key for trade. But now with oil and gas pipelines stretching from the Caspian to the Black Sea and the Adriatic, the region has become important for other reasons.