I believe that the reason Prussia is still alive is because he is East Germany.
This is actually heavily implied to be canon, but hasn't been outright said. In canon, Lithuania gave Prussia advice about living with Russia, stating that he (Lithuania) has lived with Russia before, meaning this was Soviet-era, and most likely post-WWII.
There are also quite a few differences bewtween East and West Germany. West Germany is richer. More woman work in the east (this is left over from Soviet rule). East Germany also consumes more preserved foods. Even their traffic lights are different, with East Germany's depicting an Ampelmännchen--which has become a sort of cultural icon.
So, I don't actually think Prussia's in danger of dying anytime soon. He's just moved in with Germany now that the country is united again.
