Probably the worst possible timing for the "temporary" closure. BER is not ready yet, however the airlines are beginning to restart their service beginning in june, which of course will play havoc with those servicing TXL. SXF is a mess to begin with. A real temporary closure would have made sense in april and may, but the relevant minister was against it, precisely to keep the military side working as that is used to provide transportation to the government and other political staff.
Anyway, at the moment the closure is not a finite thing yet, it is just a possibility. One thing is for sure, the civil side will close latest on November 8th anyway, the transition to BER was planned to start at October 31st. BER terminal 1 is certified now, but is not yet ready for opening, not to mention the public transport plans to the new terminal as well as other infrastructure still needs a few months until they operate normally.
That said, in a referendum the majority of berlin inhabitants were very much in favor of keeping TXL open even after BER opens, as one airport is not really enough for the city. I know airline planners have a major headache about mass diversions in case of a closure of BER, as the alternate situation, especially close to the night curfew, is not all that easy.