In my opinion , while a 15.6-inch 5G smartphone would pose challenges for portability , would pose challenges for mobility , but people who love big-size smartphones  would find ways to adapt and make it work and it will still be a viable option for big-size-smartphone  lovers who don't need intensive computing power. 

The large 15.6 inches screen size in a 5G smartphone , could still offer valuable benefits for casual work , media consumption , entertainment , productivity , and other applications and BEST OF ALL ,  better visibility .