Bula, interesting comment you made "the greater the number of engines you have the more likely you are to have an engine failure" but you are not correct. The mathematics of probabilities - rather like the old question "Is the glass half full or half empty".
The risk of engine failure does not increase in proportion to the number of engines, as you imply. The risk of an engine failure on a four engine aircraft is no greater than the risk of engine failure on a single engine aircraft.
It's the number of engines you have left after an engine failure that makes life interesting.
And Desert Duck is right on the money. A PC12 would have infinitely less chance of engine failure than a Heron (particularly with Gipsy IV engines). But then the mathematics of probabilities begs the question: Is a PC12 safer than a Heron?
Back to your original question - two factors to consider, the power available from present technology engines (less than 100,000 pounds thrust maximum) and required to move, for example 500 passengers; and the possible additional flight time and cost required for twin engine ETOP operations.
Horses for courses..........
Woomera