I've read several articles in the past couple of weeks, both local and national, regarding the trend where the large metro areas are healthy and growing in size and affluence, while the rest of the country seems to be trending in the opposite direction. Could the same trend be happening in triathlons, especially with local sprint tris?

Where in TN are you describing? If my guess is correct, the local sprint tri scene in metro Nashville would be doing well.

The sprint tri scene is doing well in Ohio, where we have three large metro areas that are relatively close to each other.