Originally Posted by Say again slowly
You seem to want to continue with the AME analogy, despite it having utterly no relevance to a discussion about a PPL training flight.
In many ways they're very similar. Both the revalidation requirements and the medical requirements have been considered at length by committees of experts, who have consulted and debated the pros and cons of different measures and come up with clear requirements which have been written into law.
If the CAA decided to put in LASORS something like "The function of the medical examiner is to ascertain the applicant’s physical and mental health and fitness, and interject if necessary to improve on these." it would clearly be inappropriate for medical examiners to use this general statement as an excuse to start making up their own fitness or psychological tests and use them as a basis for passing or failure, even if they did think it helped them ascertain the physical and mental health of the applicant and weed out people they didn't think measured up.
For whatever reason, the regulators decided that a flight test was unnecessary, but a training flight was required for revalidation. That was what they specified in the legislation. As with the AME analogy, it is clearly inappropriate for flying instructors to override the decisions of the regulators in requiring some pseudo flight test with their own made-up pass/fail requirements, rather than carrying out the training flight specified in the regulations.