I have to confess my interest is in the concept of a Single Engine Airliner.
This has been discussed on occasions in the past, and now and again you see future designs of this nature (eg Lockheed).
I fully understand you can't actually have an airliner with one engine as the normal mitigation - low stall speed for forced landing - is not consistent with required performance.
However you could fly one engine if you had an emergency back-up. So you might have your principal engine on the centre line and then a hidden engine in the tail, APU style.
The emergency engine could be massively less sophisticated because of its rare and very short term utilsation - like a cruise missile jet. It could be cheap, noisy, polluting, dreadfully inefficient but above light. The problem would be: would all of the savings from buying and running only one super-expensive high-tech motor be entirely offset by the cost of carrying the extra engine for the life of the aircraft? If not, then to me it's a no-brainer: stop putting two identical (half-power) engines on plane when a full power one is now feasible for everything except A380.