This maybe skewed a bit, but if you go by reviews on Steam, and for example, the last DLC we got which was the Prophet and the Warlock, it doesn't even have 700 reviews yet. Tomb Kings DLC for example barely has 1500 reviews. Vampire Coast doesn't even have 1000 reviews. The base game however has 20k reviews.
So if you use number of reviews as a barometer, then DLCs haven't been selling as well as people think they have. I know a lot of people frown upon DLCs and refuse to even buy them for various games.
I understand that reviews shouldn't be used as an absolute gauge here. But the low number of reviews for the DLCs when compared to the large number of reviews for the base game has to mean something.