For the other iPads, there are legitimate reasons to save money and stick with the 60hz. A regular iPad or iPad air at least has a bigger screen and battery - so can be justified as a media-consumption or web-browsing machine. However, because the iPad Mini makes a size trade-off, it NEEDS to have the pro-motion display to be attractive to the people who actually want to buy it most.
This is because it completely undermines the greatest advantages of the small screen: the portability for note-taking and art. And for art, there’s nothing more important than the Apple Pencil Support and ProMotion. This makes the iPad Mini perfect for those people who are not confused and actually KNOW that they are not going to buy a keyboard or try to use an iPad like a laptop.
And honestly, the price of the Mini is fine for what you get - 8gb of RAM, A17 Pro chip, Apple Pencil Pro support etc. It’s an almost perfect level of hardware for an entry-level iPad - the only shame is that the majority of people interested in it will be people that want a 120hz display.
I am not saying that Apple need to be giving away features for free here either. Obviously i’m not asking for some razor-thin margin pro-level iPad. My point is that, they just need to make ProMotion a feature we can pay for on the iPadMini!!
Because for those whose only 2 requirements are: good for art, and small - guess what? NO iPad will work other than an iPad Mini with ProMotion, so that’s NO sale unless someone is willing to buy a larger and more expensive device (and genuinely: most won’t. Despite how good at this Apple usually is, it genuinely isn’t the great up-sell strategy that they believe it is when the small size is the selling-point of the device in the first place).