Your first sentence in bold is undermined by your second.
There are minor differences between the Air 2 and Mini 4. Depending upon what you consider a "feature" they could be seen as identical except for screen size. What hardware is unique to the Air 2 that prevents the Mini 4 from performing the same tasks as the Air 2? (split-window? if so, that would be screen related).
If Apple remains consistent, then they'll bring support for the Pencil (and perhaps the mag connector) to the Air. Not immediately, but will eventually roll down to it just as the Air features rolled down to the Mini.
I was just speaking on the internals of the Air 2 compared to that of the Mini 4. Having a better clock speed & processor... Granted they can do practically the same task.
But in prior history... There has never been a specific accessory like the Pencil that was made for iPads. I know that Apple tends to bring prior specs from the Air to the Mini, but Apple never had created something like the Pencil. There's gotta be some type of separation from the Air to the Pro that justifies spending 300 more for it. I think the Pencil would justify that.
But Apple surprised us when they released the Air & Mini 2 with the same internals... So, who knows.