Why are sales down? The Sonic is pretty much mechanically the same car as 2012. How much has technology changed since then? Your cell phone. You got to keep up. Even if you stick with the same platform, you can at least freshen up the engines, transmissions, etc. The 1.8L is still sold with 5 speed manuals!
For some of us, the fact that the Sonic has Not mechnically changed is the very reason we bought it.
The 1.8L non turbo engine with Automatic 6 speed and the 3.47 final drive ratio, when properly broken in, is plenty responsive and fun to drive and can get 35MPG to 40MPG on the highway. It has much higher horsepower and torque than the 1.6L and smaller versions of the engine that Europe, Asia, and other parts of the world get in there Sonic versions.
The 1.8L is multi port injected and naturally aspirated. Has been in use in Europe and other countries since the early 2000's. The "family" of engines has had many refinements over the years and is basically very durable with only a handful of easily repairable parts.
There is such a thing as beauty of simplicity and the 1.8L engine exemplifies that. There certainly is a place for the higher tech and complexity, but with those things also comes the initial price as well as cost of things that can go wrong. Look at the number of problems the 1.4L has compared to the 1.8L. I won't list them here, but they are here in the forum and on the rest of the web for anyone to find.
Now if you want newer and higher technology, take the 1.4L turbo engine, add gasoline direct injection (GDI), Stop/Start engine function, etc, and you will also have more issues such as intake valve coking, even more PCV issues than the 1.4L in the Sonic and more.
In my estimation, the technology in most of the latest modern small engines is made to last a maximum of possibly 5 years or 50,000 to 70,000 miles before the maintenance and repair issues commence. If you look at what is covered under the 3yr/36,000 mile and even the 100,000 mile Emission Systems warranties, you will see many parts that are no longer covered as in past years. One can only wonder why. My view is that there are so many more "wear" items as well as the choice by car makers to keep costs down so less durable parts are used in many more places.
There are more things to go wrong with complex engine technology and some of them are just designed to last only so long. If an owner is not aware and keeps on top of signs that a part may be worn or failing, it can lead to premature vehicle demise. There is less wiggle room on turbo, GDI engines when a problem starts to surface. Too many people ignore changes in vehicle performance, "funny" noises, or even a CEL and then wonder why the car suddenly dies.