I'd like to pose another question. Is there even such a thing?

There are successful artists, and then, there's all the rest. I think the expression "sell out" is more about the jealousy of the ones who haven't been nearly as successful.

For example, are the crop of country artists, especially the guys classified as "bro country" sellouts or have they found a successful formula and are simply riding the wave and making money like any other good businessman would do regardless of the line of business?

That's my take on the expression "sell out" in referring to a musician, in a nut shell. Obviously more could be said and written but that suffices for now.

You can find my music at:
www.herbhartley.com
Add nothing that adds nothing to the music.
You can make excuses or you can make progress but not both.

The magic you are looking for is in the work you are avoiding.