Hi Kimballistic,
I do use the GFX for landscape, and it's fantastic, but there are times where a lighter load is necessary, or desirable. Additionally, the A7rIII has IBIS that will work with the multitude of Leica, Canon, etc lenses that I keep in my stable, thereby reducing or eliminating the need for a cumbersome tripod. And, of course, there are non-landscape photographic opportunities where the a7rIII excels. Between the two, for my purposes, I have all bases covered.