A significant issue is to quantify human performance. The Max certification assumed (or was persuaded to assume) that the pilot would be capable of managing an abnormal situation (without guidance and training, etc).
Conceptually this is a continuing issue in all certifications which require expert judgement (normally manufacturer); it challenges the ideas of inservice HF expertise, and if the required performance (the judged level of safety) can ever be quantified.
It's a bit subjective. But, the design requirement:
FAR Part 25.101:
........(1) Be able to be consistently executed in service by crews of average skill;......
Does specifically exclude above standard pilot skill and attention for in service flying. Yes, it can be a bit of a variable to have a very practiced company test pilot limit the application of their skill to "average", but, if in doubt, the test pilot confers with the authority in advance of a finding of design compliance.