Southern: Train cuts assume demand will fall
The South London Press has reported the threat to cut services along the Brockley route. You can read Michael Stringer's full article here, but he's successfully extracted a statement from Southern about why they're planning these cuts.
As an explanation, it leaves a lot to be desired, failing to say why a different service pattern is appropriate. There is no mention of any of the programmes they mention actually causing disruption which necessitates a service reduction so we can reasonably assume that they won't.
Instead, what this answer boils down to is that because there will be longer trains to London Bridge and more trains on the East London, they think they can get away with it.
Even if that were reasonable - which it isn't - it fails to explain why service reductions should take place before the new infrastructure is in place. Southern should not be able to reduce services unless and until the new trains are operational and they can demonstrate that demand for their trains has fallen.
Here's the statement: