G. Princip was one of many assassins that appeared in Europe before 1914 (and after WWI). Empress Sophia was killed by Italian anarchist, as was king of Greece, French President, American, and so on. If there was no G. Princip, there would be another assassin. However, there is a huge difference between Austrian treatment of Serbia and Italy in both cases. When Sophia was stabbed to death, Austria did not declare war on Italy because of this incident. Serbia was isolated after coup of 1903 and relationship between two countries deteriorated. Prior the coup and decimation of Serbian dynasty, the relationship between Belgrade and Vienna was good. Draga Masin was Austrian-Czech heritage, as was Zofie Chotek. Russia was under leadership of weak but aggressive Tzar who wanted score a military victory after being defeted by Japan. It is well known that Russia had infiltrated Austrian War ministry and had knowledge of all miliary planning and defenses in Galicia from the highest circle of the command. Russia felt it was excluded from Balkans and only country it could successfully threaten on the western border was Austria. Romania was allied to Germany as did Bulgaria. Both Balkan states had treaty with Central Powers. Romania switched sides because France promised a chunk of the Hungarian territory. Russia was determinated to break the German control of Balkans it had since 1878. The Austrian shelling of Belgrade gave Russians excuse to attack Austria even without declaration of war. Austria-Hungary declared war on Russia week after it was invaded.
Czechoslovakia and Yugoslavia were two different cases and totally different entities, economically, socially, culturally, and politically. Yugoslavia was dominated by Serbia and had all state infrastructure, dynasty, ministries, and international recongition before 1914. The Serbian monarchy extended control into the western Balkans. The Yugoslav state was only held by good will of Tito under duress of Soviet fear. Czechoslovakia was created from the scratch, but it was still a smaller version of Austria-Hungary. The laws, the political system, parties, judicial structure survived 1918 directly from Austrian system until WWII. The criminal law was in effect there from 1852 until 1950. The only difference from the past was the democratic structure of the Czechoslovak presidential republic modeled on American system. First lady was American from Brooklyn. Both countries, Yugoslavia and Czechoslovakia collapsed in WWII and there was very little will to keep the federative republics together. Slovakia wrestled much of independence in 1968 after Soviet occupation of Prague. Eventually entire generation grew up feeling foreigner to the other entity when Czechoslovakia collapsed in 1992.
I see WWI as crossroad of several political waves that held Europe for several decades and they crossed and intersected at the same time. It was not coincidence. Polish and Czech population wanted independence as had Hungary since 1867. Extensive national struggles occurred in both territories before 1914. In Poland they had sign of a civil war like in 1863. Bohemia almost had one in 1890's which was avoided by declaration of Marshall Law. It was also possible that after the death of Franz Joseph, Ferdinand would end alliance with Germany. He wanted orientation on France as was the case in the 18th century. If the war did not happened between Germany and France/Germany and Russia; it would be between Germany and Austria as it happened in 1866.
Last year I was at the castle Konopiste on outskirt of Prague. It was a residence of Ferdinandand his wife Sophie before their death. Decades after the 1914 assassination, the relatives of G. Princip and grandchildren of Ferdinand D' Este met there to close a history that changed life of so many people.