A cynic might suggest that they always intended the building to come down so that they can build a high density modern block. "Oops, it's unsafe and we need to demolish it" smells a bit like "Oops, it caught fire" to me. I hope Lewisham planners have the legislative power to force them to reinstate like-for-like (externally at least) but it sounds like the developers know how to play the system to their best effect.