its pretty obvious adsl will never be withdrawn where there is no newer tech to replace it, that alone would be a disaster for the government, so if BT were to say eg. they find it unviable to continue supporting it, its highly probable the government would step in to lean BT the other way eg. subsidies.
In areas where newer techs are available such as FTTC, it is possible but if it ever happens then the users would probably be bumped up to the newer tech.
BT arent going to just disconnect users en masse.
BT Infinity 2 Since Dec 2012 -
BQM