I guess you don't understand what is meant by main target market. The point is, people like you who upgrade at every iPad iteration are in the minority, and are vastly outnumbered by people who upgrade much, much less frequently.
If you like to upgrade this way, they're more than happy to sell new iPads to you, but their model iterations are not specifically designed to capture this smaller group of customers.
In addition, the vast majority of iPhone users do not upgrade every year either. In fact, the average upgrade period for iPhone users is now every 4 years. Yes, 4. And that's average, not the longest. Even if you went by previous averages, it was still much, much longer than a year, at about 3 years. Furthermore, IDC states that for the 2019 year, they expect only 16% of the installed iPhone base to upgrade. IOW, your statement that "iPhone users tend to upgrade every year" is just flat out wrong.
BTW, I actually agree that iPhone users likely tend to upgrade more frequently than iPad users. However, if only 16% of iPhone users will upgrade this year, what do you think that means for iPad users?