"Its easy to change a battery!" kind of misses a main point in my view.
It is hard to drop a battery that is locked safely in the camera.
It is hard to short out a battery in wet conditions, when it is locked safely in a weather sealed camera.
It is hard to get dirt or something in the battery slot of the camera, when it is safely locked and closed.
Changing a battery or a card becomes a point of failure. It becomes a time where something damaging or bad is far more likely to happen, because there are now loads more possible things that have some non-trivial probability of happening.
Comparing the A9 with a grip and some spare batteries to a 1DX MkII and declaring it a complete win is ignoring a lot of the features and qualities that make people want a 1DX. Namely durability and reliability.