Not clear to me that you'd design a commercial turbofan aircraft with one engine if you eliminated reliability requirements. You need to feed air to the engine and, to the degree you have wings, might as well have 2 engines.
The idea that the engines need to sit on the wings like they do is something seems important, because it's so often done, but is actually not necessary. You can put a single engine in the tail, or on top of the cabin, or on the top of the cockpit, or underneath. They go on the wings because when you need two (for redundancy) it's a good place to put them for balance, and because it makes them nicely accessible for maintenance.
Of course military aircraft have different requirements (and many have just one engine in the tail). Certainly I'm not enough of an aero/astro engineer to know if, positing a perfectly reliable engine, you'd mount just one somewhere on the fuselage.
Not sure what you mean? I didn't say it was always done. In fact the point I was making was that it wasn't always done. I gave my own counter-examples. It's just when people look at modern planes they always see two engines and think that's the way it has to be. It clearly isn't.
The MD-80 (and 727) were old turbojets though. Probably a lot harder to come up with a practical tailmounted design with a turbofan although I'm open to it being possible to mount one turbofan above the fuselage.
