I agree that the public do not give a toss about performance metrics. However the industry does and it likes giving itself a good slap on the back if it does well. The performance metrics therefore fall over because it isn't really representative: consider the unlikely situation that everything on Saturday ran perfectly apart from that - that'd be 100% on Right Time, etc, but it isn't - because passengers journeys have been affected. The industry does terribly when it comes to thinking and experiencing the railway actually like a passenger.
The patting itself on the back thing is part PR and part political. It is just a fact of life. I don't like cringy PR or political point-scoring but I do appreciate they are important at promoting rail travel as part of a wider sustainability drive. Lots of things need improving across the industry, including changing mindsets and many things are slowly improving, but there is a long way to go and it can be argued some changes should be implemented quickler. On the other hand the industry's hands are tied to a certain degree as changes cost money and the latter is not in plentiful supply, and subject to prevailing government agenda.
All metrics come with their own nuances, whether it be PPM, T-3/5, full/part-cancellations, excess waiting times, lost customer hours, etc. There isn't a one-size-fit-all measure that can reflect every customer's experience, be relatively straight forward to calculate, and easy to present/track. That's why I alway say take performance metrics with a pinch of salt. Use them as a rough guide but when you start drilling into individual days or individual journeys you will always find incompatabilities and inconsistencies, not to mention they are not necessarily
that accurate to start with. Even on days where headline figures are reporting 100%, some customers may have had very poor experiences due to factors other than punctuality.