Will we ever see a single-engined airliner?
There are two major problems I see.


Even if you have an incredibly reliable engine how do you prove it is incredibly reliable. I can't seem to find figures for the target IFSD rate for the current higher ETOPs levels, but I find figures for early ETOPs of one per one hundred thousand hours. Lets say you improve that to one per billion hours, how exactly do you demonstrate it's that low? fly for a billion hours? Thats a hundred planes flying for a thousand years. This is why redundancy is the route taken for many safety critical systems, you can argue that the individual components are merely very reliable and then argue that the chance of two of them failing at once is extremely low
Where do you put the engine? If you have a single engine you want it on the centerline. That is not such a problem for props or low bypass turbofans, but it's difficult for high bypass turbofans.


Because of these two factors I doubt we will see airliners move to a single engine configuration unless we see fundamental changes in airliner design and regulation.