It's not the availability of technology. Apple chose not to update the iPad mini 4 or 12.9 last year and the iPad Pro 9.7 didn't have all the non-calling features or tech of the iPhone 6s, which was the current phone when that model was announced -- most notably 3D Touch and Raise To Wake functionality. The iPad mini 4 doesn't have a Touch ID sensor even though that technology existed when it was announced. Those are all management decisions.
Not debating your points, Chupa Chupa.
What is clear is that the feature-sets are staggered and not synchronized -- those sets appear haphhazard from the outside-looking-in, and unexplained. Why?
Hell, who knows? Not even the Shadow knows! LOL!
But a trend is apparently coming thru (if you believe the rumors): displays are in transition to edge-to-edge OLED, due to physical attributes of flexibility and power efficiencies. That is what I meant with technology in transition and availability.
On the iPad Mini 4, my wife loves it for its light-weight and small size, used for email, iMessage, and FaceBook posts. I am never granted access to it, unless it is for technical reasons (LOL!).
My wife never misses the TouchID sensor as this iPad never leaves the house. (As a matter of fact, she bitches about the calibration of the TouchID on her SE.) And, I do not see her wanting to update to an
incremental iPad Mini unless the new version inherits an edge-to-edge display, while preserving its 7.9-inch footprint and weight.