People used to design for desktop first, then port to mobile as an afterthought. Originally, there was only desktop, and mobile was a completely different and separate thing. Then mobile got more capable, and by necessity (since everything was designed for desktop) mobile tried to be a small desktop.
Mobile was limited, and trying to be a small desktop didn't work out that well. So people switched gears and said "Well, let's treat everything as mobile first, and let desktop adapt because it's more powerful." So we began the mobile-first trend.
But in the meantime mobile has gotten much more powerful and less limited; today many mobile devices have higher resolution and more powerful cpus than the old desktops had. The screens have also gotten a lot larger than they were when mobile-first started.
So, I think we'll either wind up with a "tablet-first" trend, designing for the middle-ground and making both the smaller mobile and larger desktop adapt, or we'll go back to recognizing that mobile, tablet, and desktop needs are different and we'll design accordingly for each, but now with a better understanding and better toolset for minimizing the redundancy that can occur with split designs.