Apple has to keep supporting the iPad Mini 4 for several more years since they are still selling it now. Imagine someone buying it now and in one year when iOS 13 is announced, it's not supported? That would not go over well.
Apple has really dug themselves a hole by not refreshing their products annually, and keeping products around unchanged for 3-4 years. People expect a minimum of several years of software support from Apple, and as they continue to have a line up of products that use 2,3,or 4 generation old hardware, they have to continue to support those products for many years.
I really cannot figure out Apple's strategy with most of their forgotten products, like the iPad Mini. It seems like it would require very little resources to keep it a generation behind in processor and RAM. If they just put an A10, not even an A10X, with 3 gigs of RAM in the iPad Mini design as is, many people would be happy and satisfied. I know Apple likes to be considered innovative, so just incrementally updating something annually doesn't seem to be their game. But what they are doing now by not even trying to update a product and continuing to sell it at a premium is even worse.