If an FTTC cabinet is at full capacity, you're normally able to order ADSL and wait for an FTTC port to become available.
How will this work if your local exchange is due to close in the future (and ADSL/Voice services ceasing)? Will BT/Openreach only close exchanges where adequate FTTC or FTTP capacity is available for all premises to take up?
Exchanges carry lots of other services as well as ADSL, so exchange closure can only happen when its practical to offload them.
ADSL is going to be around for quite a while yet, in many cases its prided from a MSAN, which is part of the core network.
https://en.wikipedia.org/wiki/Multi-service_access_node