$400 (discounted) + tax is not nearly $500 USD in the same way that $250 (discounted) + tax is not nearly $350 USD.$450 + tax is indeed nearly $500.
If you are that budget constrained then by all means get the 9, but some people might like the benefits of the 10 and consider $150 an acceptable upgrade cost.And in the past 12 months, it's been much easier to find the 9 on sale for $250 than it has to get the 10 on sale for $399. Even on sale the 10 is 60% more than a sale-priced 9. $150 is not insignificant.
Sure, that might be an additional reason to choose the 9. Or someone may not have a bunch of old accessories beyond a case and a few Lightning cables which are probably beat up after several years of use.Also the 7/8/9 share the same case size, and you could even have accessories from an old Air or iPad Pro 10.5 that would work all perfectly fine.
Not everyone is driven by the lowest price when it comes to iPads. If they are, the 9 is a great iPad.If you're making the choice between spending and extra $150, and when you may already have accessories on hand, the 10 is a much harder choice.
My actual point was that if you are going to compare prices, compare msrp to msrp or discounts to discounts.