Interesting thought. However, the design parameter of engine size and number is dependent on available technology. Imagine if, back in the early sixties, an engine of Trent size was available instead of an RB211 size....what would a 747 have looked like with that type of power availability. Same size on two engines or supersized on four. Surely, the design parameter of excess power equals performance is more important than how far you can stay in the air on one. The job is to get the weight to the design cruise level economically. After that, the envelope gets looked at on how far can you fly on whats left at lower altitude on higher specific fuel flows...isnt that what ETOPS was all about...not running out of fuel rather than the stats of a catastrophic failure?
If the stats become king then...as witnessed by the Hudson River event...you would not launch because it CAN happen!
Obviously above my pay grade but isnt that the gist of ETOPS?