This is why iPad sales are 'struggling'.
The cart went way past the horse. PERIOD.
It took FIVE YEARS for the iPod to sell 100,000,000 units.
It took FOUR YEARS for the iPhone to sell 100,000,000 units.
It took only TWO and a HALF years for the iPad to sell 100,000,000 units.
It is by far the fastest selling Apple device to sell 100,000,000 units. The growth in the first 2 years have been beyond explosive. Now we are see normalization to the mean. The iPhone was given five years to mature to a point where solid growth stabilized. Technology was able to catch up and overtake consumer demand. The same thing has not happened with the iPad. It has sold and grown so fast that technology has to catch up to make future iPads appealing for repeat buyers.
There is nothing to worry about. Again once the technology catches up to demand (TouchID, larger screen, more ram, foldable screen, ect) we will see steady growth. The rate the iPad was growing was unsustainable. By next year I expect the iPad to grow at a nice 15-20% YoY each quarter.