Old Fella: In fact, you don't have to "accept" CASA is the "final arbiter". The current system is the result of a choice. A different choice could be made.
The rules could just set the objective standards and expert medical practitioners and specialists could be left to make the decision on whether the candidate meets the standard.
Even under the current system, the AAT can change CASA's decision and, in the case of medical certification in particular, a far greater proportion of CASA's decisions are being changed by the AAT than was the case e.g. a decade ago.
The recent change in approach to colour vision deficiencies demonstrates to me that the increasingly intrusive and disruptive activities of CASA Avmed are not based on any credible data. All I see is some relevance-deprived zealots on a medical crusade, wrapped in the flag of 'aviation safety', doing more damage than good.