It will be indeed the end of an era. People will have to find new hobbies and other things to do with their time.
Most of the vast knowledge people have acquired relating to DSL will be completely worthless, and as we simply can't chose to forget this knowledge, or reclaim the memory space it's claimed in our neurons, this will be a depressing thought for many. We'll only have access to the same troubleshooting steps as Joe average (turn it on and off), as opposed to the multitude of xDSL related troubleshooting steps we can do now.
History has shown us that there will be some that will cling to xDSL for as long as possible, similar to when some folk refused to stop cooking with wooden fires when electic/gas ovens came about. Or maybe those choosing to watch their tried and tested black and white TV's instead of upgrading to colour. The nail in the coffin will be when xDSL is completely turned off, hopefully BT set this date as soon as FTTP/4G/5G are sufficient to cover everyone.