Let's imagine secession wins with a 52%, and everybody decide to divide Spain. This means 48% of the people are going to be taken their Spanish passports and citizenship off, and they become just Catalans. This opens a can of worms in terms of stability of the region, which may very easily end up in another civil war, like the one that happened few decades ago.
Another solution would be to split Catalonia in two regions. This definitely will end up badly, taking in consideration how resentful Spaniards have been through their history.
I do not know if this quote is originally from Bismarck as some people claim, but he rightly said:
"I am firmly convinced that Spain is the strongest country of the world. Century after century trying to destroy herself and still no success’"