Perhaps it might make little sense to the consumer to upgrade every year. You could say the exact same thing about iPhones. I'm not saying it IS worth it for Apple to update every year-- just saying we don't know that it isn't.
But that is what has been shifting the tablet market space, are those consumer choices. We would also know by now if they were planning new Pros, and so far the analysts are suggesting not till early next year based on their production line knowledge. Further, Apple has also hinted or outright told us their plans for iPads are a bit different since 2014 during their shareholder meetings. Apple has admitted that iPad customers update their devices less frequently than iPhone customers and that has played a role in what products are appropriate to upgrade (i.e. iPad Air 2 to 3 wait, or Mini 4 to 5 wait, or even iPad 4th gen to 5th gen wait). These changes do not happen overnight, which is why over the past 4 years they've shaped their product line up differently vs their phones. The upgrades are not always on a certain, predictable 12 month cycle.
And it makes sense why, beyond the consumer side of the coin, as well - The tablet space itself is a bit different from the smartphone space. Apple is competing on a different scale there (a smaller scale, but also one they essentially hold a monopoly on), whereas phones are a much larger pie, more global (especially more prevalent in developing nations), and most significantly are also tied to carrier contracts (that may or may not be subsidized but certainly offer incentives to renew those commitments). There are multiple industries that independently make billions from phones, and are thus very invested in getting consumers to continue the upgrade treadmill on a regular, predicitbale basis.
The same is not true for computers or tablets outside a small minority of consumers who do tend to upgrade those products on a more regular basis. The tablet space is more like the traditional laptop/computer space, where there is less incentive to upgrade those products. The percentage of tablet users who upgrade every 1-2 years is much smaller than the percentage of smartphone users who do the same. Many people still use iPads from 2014 or 2015 but have replaced their phones 2-3 times in that same time period. And this is why Apple seems to have stretched their 12ish month iPad lifecycle to a more 18 months life cycle or more, whereas smartphones and computers are still getting more regular and frequent upgrades. They are reusing more production parts and thus decreasing R&D costs, Production line costs, product order costs etc...meanwhile making it both cheaper to the consumer and more profitable for themselves at the same time.
TLDR; We do know with certainty, or we can reasonably infer, based on: future product line leaks, their quarter earnings, their shareholder meetings and quartyerly reports especially over the last 4 years, and how they've shifted their production strategies with their iPads over the last several years
(Now watch them release the A13X iPad Pro in October
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) haha