A rational line or reasoning might run something like this:
1. Is double-engine-failure the only "single" event that would inherently bring down an aeroplane when over the ocean? If no then:
2. Is the probability of a double engine failure higher than the probability of those other events? If no then:
3. Why are you fixated with engine failure when you're prepared to accept a similar probability of crashing or ditching in the ocean as a result of more probable events?
It is my understanding that the whole ETOPS thing came about because it dawned on people that they were demanding higher reliability from the engines than they were from other items which were just as critical to continued flight, and that's just not a rational thing to do. So instead of saying "Thou shalt have at least four engines*" they said "Thou shalt have a probability of failures (engine or anythingh else) sufficient to prevent continued flight of less than x".
PDR
* What's so magical about the number 4? WHy isn't it 5, 6, 7, 10, 20, 100 or WHY? That's what demonstrates the whole concept of dictating a number of engines rather than a system reliability performance is just irrational