Glueball, you haven't answered the question. Would you prefer to be on a 747 on 3 engines (ie, it has magically become a trijet that used to fly this route) for 10 hours, or on a 777 over the Pacific wastes on 2 engines, with the likelihood of 3 hours cruising on one engine to get you to safety if you lose another.
I know with the enormous redundancy built in to the 747 which I would like- even down to 2 engines, you still have 2 more. You could even lose another engine- not at all pleasant, but you would still be flying.
Fathom- you haven't thought this through. If our B-52 loses #4 (on the left wing), he is (according to your logic) 'reducing his margin'. Should he scrub the mission or carry on and ignore? Where do we cross the line from dodgy ETOPS operations to safely redundant 4 engine ops? We go from twins (land at nearest if engine out) to Trijets (think about landing soon if engine out) to 747 (carry on if engine out, but watch out for 2 eng driftdown and monitor diversion airfields).