Late delivery of a replacement train is about the only valid reason to extend the operation of a non-compliant train. Perhaps also a train that is in the process of being adapted not yet being finished for reasons outside its control.
We've been preparing for so long now, and I've said from the start that there can't be extensions through lack of planning. If a TOC hasn't taken the steps to get stock ready, they should be banned from using the non-complaint stock and fined heavily.
If it was always clear that would happen, things would have been done on time. Instead, I get the feeling that the industry has dragged its heels in places with the view that they could just be late. I also feel that it has been the belief that nobody would dare ban stock from use, as it would punish rail users, and so the deadline was not to be taken seriously.
Fortunately, it seems that not everyone has thought like that and we're seeing some real improvements with new and modified rolling stock.