How many southerners would have seceded again in 1870 if God or Aliens came and gave their approval? 1880? 1900? What about 1950? How many Serbs, Italians and Romanians would want to leave by 1950? There is a problem when you to say that they 100% would just because of want their parents or grandparents wanted. Cause these people are not rural foke anymore, they are urban foke who now outnumber the countryside around them in terms of population.
If Austria-Hungary had lasted 50 years and undergone a demographic transition the most impressive thing is going to happen to the cities, which will be holding tens of millions of people. These are going to be muntiethnic, cosmopolitan, metropolises. Every city will have a German, Slavic, Hungarian, Italian, and Romanian quarter were people mingle. Every. Single City. People from diffent ethnic groups working together everyday. People living in them are not going to have the same attitudes towards nationalism that existed at the beginning of the century.
An Austria that survives to become a urban country connected by its cities will be as different from the old rural Austria as the pre civil war USA is to the mondern day.