An iPad is large enough for both face and touch ids - so give owners the option.
If they follow the same pattern they followed with the phone and watch, we would expect to see reduced bezels. And with reduced bezels comes the removal of the button. If they included both, they wouldn't be able to offer larger screens in the same sized package. You'd need a bigger iPad to get a bigger screen.
So I don't agree that there's room for both and really hope Apple doesn't do such a thing. I want the bigger screen and don't mind FaceID. So I'm looking forward to a FaceID iPad with a larger screen in the same comfortable size I'm used to.