Many of the world's most successful airports are not capital city operations (...) FRA, MUC (...)
. Don't use German airports as an example though, this is a very long story and has mostly historical reasons (post-WWII). E.g. Munich was the capital of the Bavarian kingdom. Frankfurt nearly became the capital of West Germany, and is the financial powerhouse. Also, Berlin (with a little help from the Federal Government) is building the brand-new airport, which will overtake Dusseldorf, and maybe on the long run even Munich. Without WWII and the Wall, no doubt Berlin would have Germany's biggest airport. Stuttgart would be a remote village, all manufacturing would be in Berlin etc...