Two times I have had my coolant fall slightly below center. The indicator is not linear, which is why we say, if it is even slightly left of center, you have a problem. The first time I suspected it was the sensor because it came on quickly and it would wander up and down on the highway. I took a chance and replaced only the sensor so that I could better report back to this forum.
I was right. It lasted six years with no further problems.
The second time was recently, and it fell below center and didn't waver. I suspected the thermostat, but because I had a good idea what a failing sensor looked like already, I replaced both.
So out of two times, once was sensor for sure, and one was likely one or both. So in my case, sensor failure was at least 50% of my occurrences.
The reason we always advise to replace both is not that they are side by side. It is because there is no way to tell which it is with 100% certainty, and the consequences of not fixing it quickly are expensive and severe. If either the sensor fails or the thermostat fails, the PCM pumps in raw fuel thinking the engine is running cold. This results in a cat converter that is going to clog in relatively short order. We have had LOTS of members having to replace cat converters at some point in their lives ... and every single one reported that at some point in the past, the engine temp was below specs. It does not even need to be low enough to trigger a code; driving long term with that gauge one tick below center WILL cook that cat sooner or later.
I fix mine right away. I test the performance of my vehicle every now and then, and the highway acceleration (the most telling indicator of cat failure) has not dropped since the day I bought it 12 years ago.
THIS is why we advise changing both.