And you would be correct. Both Tempelhof (THF) and Tegel (TXL) are slated for closing within the next few years, even though there are vocal movements to save both airports.

The idea is to turn Schönefeld (SXF) into Berlin-Brandenburg International - a new mega-airport for Berlin and its surroundings. It's a pity, because both TXL and especially THF are extremely convenient since they are close-in city airports, but they are just too small and too far above their intended capacity.

As for FRA's success - it really is about location, location, location. Millions and millions of people live within a two hour travel time of FRA, it is very centrally located in Germany and in Europe and the major industrial zones around Stuttgart, Mannheim and in the Ruhr region are all close by. The banking industry in Frankfurt generates a lot of business travel. And, as others have said, the high density of US military bases in the surrounding area back in the day helped a lot.