Dr Strangecab,
Without deep insight into what was going on in the minds of those involved, things aren't so easy to brush off.
Although on a different aircraft, I too have held reds and greens. I too am a CEng. I have also been involved in the RTS process. Having reds and greens on type does not give you all the knowledge that you need to assess whether or not a fleet of aircraft is safe to bring into service. Nor does being a CEng. Having reds and greens only gives you some experience, being a CEng only the opportunity to exercise your duties with responsibility.
Yes, it is the responsibility of the Duty Holder as to whether they accept the risk. Obviously they felt that the case was not made. What 'other Boeing customers' think may be of little relevance if Boeing themselves cannot demonstrate that their product is safe.
sw