There's probably a number of reasons behind it.
The PRM derogation for the 153s around the country appears to be until the end of this year. I can see that being extended again, but I'm not so sure about the Pacers being extended. The WG and TfW want rid of them, but they are filling the gaps. Had covid not been around resulting in dropped passenger numbers then it would be a huge challenge to try and run a decent service without the capacity provided by the Pacers.
Apparently there might be at least one 769 in service from the Dec timetable as opposed to the planned 5 by now. That is a milestone in itself which will help ease the pressure. Also if, as discussed on a different thread, more 153s are coming then hopefully by Dec the Pacer usage will be minimal, if none at all by Dec timetable.
I would imagine that trying to get parts and maintain the Pacers is now also becoming an issue as there aren't many left and I'm sure parts are hard to come by now. So the less usage, the better.