Signals to the end of an era, smartphones have matured and are becoming ubiquitous like laptops. Moreover, the most profitable smartphones belong to Apple which held 86% of smartphone profits generated in the industry in 2018. Samsung was competing more in the mid to affordable segments so it was bound to start losing once others started catching up, especially since its Chinese competitors are basically giving up profits for growth in the affordable to medium segments.
https://appleinsider.com/articles/18/04/17/apple-grabs-86-of...