One of my work mates had a team brief a few weeks back and one guy asked what is going to be done about the state of some exch MDFs and some manager told him nothing as they would all be gone by 2020 as we will have a full FTTP network ! LOL there are two chances of that happening , slim and none and slim has just left town .
Of course, its not going to be a full FTTP network but the days of the PSTN are numbered - and as others have said the smaller exchanges will eventually be closed.
BT recently said:
"Today, customers get a telephone line as a pre-requisite to having a broadband services. With single order broadband, customers will no longer get a telephony service with their line, they will get a physical line over which broadband is delivered. From 2018, we expect on the BT network that if the customer also wants a telephony service, there will be a pre-requisite of having a broadband line and that by 2022 we anticipate that the PSTN voice service will be withdrawn."
and ISDN etc. is due to be withdrawn by 2025..