Your speculation is no different nor any more valid than those speculating that the iPad will be updated every other year.Why do people think that Apple will only be updating the iPad every other year? The only reason they have not updated this year is probably because the iPad Pro being released.
At least the "every other year" is derived from some data. iPad Mini 2->3 was a non-update update. There was no iPad Air 3 announced this year. The iPad Mini 4 released this year has virtually identical specs as the Air 2 (released last year).
Sales rates for iPads have softened... analysts point to market saturation and longevity of ownership as contributing factors.
Seems like a reasonable conclusion to draw that the iPads will be have bi-annual updates... the Mini and full-sized alternating years so that every year at least one iPad model gets updated.
This is different than iPhones where a majority of people are on 2 yr contracts. At any given time, on average half of iPhone owners are eligible to upgrade. That is sufficient incentive to update the iPhone every year.