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To another person, IF it can be tentatively assumed after investigation that contributory factors external to the aircraft itself such as crew and fuel can be discounted and factors specific to the aircraft such as the independence of the two engines is as certified, then the rollback of engines on an ETOPS aircraft within seconds of each other and a failure to respond to further requests for power might be seen by many as either a statistical fluke (if I may, equivalent to a sperm whale finding itself freefalling to earth with a bowl of petunias) or evidence suggestive of a flaw in its certificated design/performance. I accept the assumptions here are crucial.
I may pick at your arguments somewhat, albeit they have many sound qualities. Remove the ETOPS argument above, as the issue equally applies to all aircraft operations.
Also to be pedantic, the term rollback has historically been used to describe a different malfunction scenario so perhaps we should only use the words released to date by the investigators.
continuing in pedantic mode
There is no such thing as a fluke in statistics, as their use is only a means of communicating natural occurences (combinations)
I remind that neither the designs nor certification imply all is perfect and free from catastrophic occurences. There will allways be the rare occurences of combinations not forseen nor even recognized after the fact, that will exist in aviation for relatively short periods of time (measured in risk per flight hour).
Some risk of unknowns is acceptable even some risk of knowns is acceptable over controlled periods of time. The issue is to be certain that the level of risk is within acceptable bounds and this is done by continued dilligence and investigation of potentially related events (either in history or today's)
So in my opinion groundings or flight restrictions are not evident at this time, but continued dilligence, (investigation and understanding) is appropriate.
show me the data and I will recommend the action