Sounds like a little competition is required.
Berlin Airports controlled 3 airports:
Tempelhof - a business or city airport which could have survived and operated in the London City commuter niche. Unfortunately, it has already CLOSED.
Tegel - home to Air Berlin, and regional base for Lufthansa Group. BURSTING with little new investment due to the 'imminent' opening of BER.
Schonefeld - former GDR building which turned into a LCC hub for Easyjet and Ryanair (and previously Germanwings). BURSTING with little new investment.
And of course, the sleeping White Elephant.
With private competition rather than local political interference and a monopoly position, Berlin like London could have had 3 independent competing airports offering differing products for alternative markets. Alas only 2 airports remain, but if Berlin Airports were forced to sell Tegel to a private operator, the cash gained from the sale may eventually push them to finish and open BER.
BER would then become a hub for network carriers, offering long-haul connections.
A natural home for Air Berlin and LH group.
TXL would then remain open, run by a new owner and offer an alternative product focussed on short-medium haul routes.
As to which airport would become the base for easyJet, Ryanair, Norwegian etc would be subject to competing commercial agreements. As is the case with LHR, LGW, STN and LTN.
I'm amazed that free market competition can still be stifled within an EU country.
But then again with ADP controlling all Paris Airports, ANA controlling all Portuguese airports, AENA controlling all Spanish airports, perhaps a free market isn't what Europe is about these days. Little seems to have changed from the days of the GDR.