I suppose gesture use is fairly dependent on hand size/finger length. I have a 5.7 inch display on my phone right now, and swiping down from the top to reveal more notifications (on Android) is pretty simple to do with the thumb. It just takes a slight shift in my grip. The only thing I do that requires two hands is occasionally typing, but even that I frequently do one-handed. I realize that I don’t tend to grip my phone so much as let it rest on my fingers, as this is what allows me to easily use my phone one-handed and have my thumb reach everything. So a “grippy” material would indeed be beneficial.
In short, I think reaching all parts of a 5.5" display with the thumb is not at all unreasonable, and is certainly physically possible, but commonly used features should be reserved for the bottom part of the screen, as is planned. Grippy material is good for facilitating such one-handed usage.