You have things backwards. It was an economic decline that helped to motivate the collapse of Yugoslavia. If things in Yugoslavia had stayed as nice as in the 1960s and early 1970s -- the era of the vikendica and middle-class holidays in Bali, -- nationalist grievances might not have gained ground.
That's a bold claim. It was a communist dictatorship first - not sure if there is a reasonable way to compare GDP of a communist country with a normal country. Then there was an endless civil war - that's hardly good for economy too.
Today these are quite comparable to other Eastern European countries. Croatia has a larger GDP per capita than Romania, Montenegro is somewhere in Turkey's league, Serbia is pretty poor, but still much better than Belarus and Ukraine.
