My current writing teacher has 'put a head' on an issue that has been looming since the day I took up the hobby of writing. There seems to be two completely different mindsets when it comes to writing, and they are not compatible.
The first is that it would be nice for any given written work to be entertaining, but it should never be its 'primary' purpose. So the purpose of any written work is to be informative, thought provoking, emotion evoking, and so on. These people would see a book that 'merely' entertains to be a failure.
The other mindset is the one I have. I think that entertainment is a worthy goal. Not the only goal mind you. I have nothing against an informative or emotional piece, it's just not what my goal is. And truth be told, I would be happy if my writing is thought provoking, even though it's not my primary goal.
Given my goals, popularity is a good standard. If people are entertained, they will buy it. If no one is buying it, then it's probably not entertaining. Of course there are exceptions, but I'll keep writing entertaining fiction, and cross that bridge when I come to it.