I do feel that there are some very important factors relating to twin engine helo ops, especially the shared dynamic systems. I realize its
very similar for fixed wings but multi engine fw do have the advantage of largely separate propulsion/thrust units and which I believe offers an added level of safety.
One solution, from a cost perspective, would be an engine technology which allows for very high short term power demands of say up to 10-20 mins (for emergency single engine operations) from smaller engines which would offer more economic operating costs for twins.
But, the way things are going in the H&S world we live in I am sure singles will end up as trainers and private use only.
Here's a single op that recently had the hangar door closed on them:
Rescue copter loses out | Stuff.co.nz