It's normal as the game has aged the sales will drop. Perhaps will see a surge when winter sales. But i'm really surprised Dayz is still best selling after so long? Perhaps the game is constantly evolving and contents are progressively added.

Divinity Original Sins could be a one hit wonder. If Larian can reproduce the same quality and the same attention from journalists/press on their "next" game, then Larian will change. It's no doubt they will be contacted by giant corporates who want to partner with them.