Most Cities in the World would give an arm and a leg to have a city centre airport such as TOF. I can't fathom the decision myself, except to say that it was purely political arm-twisting, misguided Green activists and vast amounts of embezzled and misappropriated public money that got BBF in to such a mess in the first place. They spent huge amounts of money on land which will never be part of the 'new' SXF back in the early 90s. They forced the Berlin City Council to accept that SXF had to be developed at the expense of TOF and TXL, otherwise it would not be viable if all the short haul and business travelers used the two smaller airfields.
When you consider that Leipzig, not so many miles away, designed, planned and built a new airport in a few short years in the mid-nineties, taking a lot of the traffic which might have used SXF, you can see that the BBF already have the makings of the biggest white elephant in the western world.
Closing TOF simply shows how big business and political in-fighting can ruin a perfectly useful and usable airport in the heart of the city.
Go see the place, above and below ground, before they wire the doors closed.