Carrie co-wrote "Southbound", and included it on her first formally co-produced album - I'm pretty sure she wouldn't have done either, unless she liked the song and wanted to commit to it (she's known to be choosy about what ends up on the album, and does drop her own co-writes if she doesn't think they fit what she wants to say). While it's true that we don't know how much input she actually has into radio promotional singles, my guess would be that, at this stage of a leading star career, it would be substantial. Also, I don't think she would have performed it as her only solo at the ACM showcase (where she's both well liked, and a big draw) if she was in any way doubtful about it. In short, I think it's safe to assume she wants this song.
As I said, her career is particularly closely identified with radio (I can for example, immediately, think of three other currently prominent female singers who appear less concerned with radio, and have built other areas of major support, in Pop, Alt Country, touring and critical approval). Carrie may, indeed, be able to beat these at their own games if she so chose - but, so far, they have not seemed her main focus, and radio hits tend to dominate her signature performances, and much of her identification with general listeners.
"Southbound" does seem on anecdotal evidence, the "safest" radio song - in theme, summery mood, avoidance of controversy, catchy identification, similarity to recent popular hit material, etc. After two, very different, but relatively underperforming singles, it may have seemed the likeliest choice to attempt to reassert her leading place on radio. Her co-writers, David Garcia and Josh Miller, for example, also both co-wrote Bebe Rexha's recent mega hit - and while that may not exactly assuage some of the feelings expressed here - in practical terms, the point is likely to have been noted.
In short, there are reasons for going for this song, and if that's what Carrie wants, or feels may offer the best short-term benefit, then I feel we should do what Maddkat suggests and concentrate on the chart run. The era, as a whole, still has legs to run. - and if this song does well, that may help to increase the prospects for prolonging it.