I've often wondered why something as horrendously expensive as an F-35 only has one engine?
The answer is that there has been a small revolution in the manufacturing industry using CAD/CAM software at a new level. More complexity and more failsafe can now be built into the inherent designs with less hands-on human involvement in the manufacturing process. The end result being you get what you designed for quicker, cheaper and to spec.
But in my opinion we are still not quite there yet with human oversight still creeping into the designs in ways never considered e.g impurity levels in base materials, running a crucial oil pipe through the hot section of a turbine, software flaws, etc, etc.
Add to this equation that engine thrust levels have been increasing rapidly creating much less need for more engines under the wings to achieve a similar performance. This means bigger aircraft like the A380 can now be built out of all that extra power, but is it any safer? 747s have fallen out of the sky and often it was not because of the added safety of having 4 engines. Then their was that early model A340 which had these 4 wimpy engines and seemed to require all of the runway to leap into the air.
2 engines are good and 4 engines are better, and as long as we don't kid ourselves about an aircraft's abilities during degraded performance the 2 engine variety should continue to serve us well into the future as they get even more powerful. By then we will be arguing about having only one engine which is already a contentious issue in helicopter land.