Landscapes tend to be more challenging when it comes to fine detail. Now any modern FF camera 20mp and up can produce a competent 16x20/24" landscape print. But even at 16x20/24" some compositions may be more immersive with more MP and the right printer/paper combination. As you go larger MP starts to become very important to landscapes for critical viewers at close viewing distances.
Contrast this with portraits which used to scale pretty well to 16x20" with just 8-10mp. Sure there's more detail to record with higher MP and sharper glass. But it just doesn't seem as critical as the fine details in a landscape, so long as there are some sharp details in the portrait (notably the eyes). I felt like 6-8mp APS-C struggled with some landscapes even printed to 8x10. Never felt that way about an 8x10 portrait from those early DSLRs.
Yeah...it's easy to say that until you shoot regularly at higher MP. Then variations pop out at you while in PS or LR and you don't hesitate to crop because you've still got 20-35mp and it still looks good. That's true for all subject matter. IQ and cropping ability soundly trumped higher fps for me to the point that I'm shooting everything, even sports, with the 5Ds now. A 90mp R5s would just bring more flexibility to the table. Your crop might end up at 45-50mp which is still excellent for very large prints.