Is Air Serbia doing that well? This is a case where the JAT infrastructure may have been the best.
Air Serbia does quite well actually. They launched 9 new routes from BEG this summer out of which 7 are year-round.This summer they had a crazy charter season, I think it grew by 10%. On top of that this winter they increased regional flights to Bucharest, Skopje and Tirana.
As for a pan-Yugoslav airline, why should we do it when most republics can't stand each other? There is a reason Yugoslavia collapsed. The whole project would collapse at the very beginning when they would try to decide where to base the HQ.
Adria could have made it but they made some bad, senseless business moves which led them to bankruptcy. They could have become some sort of a Balkan airBaltic.
Of course the Nordic countries are richer than the Balkan/ex-YU ones when they never had Communism or war. So comparing them makes little sense. Long term Balkan states have far more potential than the Nordic ones because they have a larger population so more potential. Look at what is happening in Poland right now or the Czech Republic or Hungary.