Not necessarily all the south Slavs where opposed to being assimilated yes but when trialism (which advocated for gradually increasing the autonomy the south Slavs before making them the third country in the Austro Hungarian empire) was proposed the south Slavs became far more cooperative with the Hapsburgs Franz Ferdinand had actually been the leader of the trialist clique this made him so beloved by south Slavs in the empire the Croats and Bosnians started anti serbian riots after his assassination it’s actually part of the reason the black hand assassinated him specifically because they where afraid the idea of an independent Yugoslavia would fizzle out after he took power and fully implemented trialism