I think the reason the mini hasn’t been updated is the same reason the MBA hasn’t been updated. Once you factor in the cost of all the components, it’s not profitable to sell it at its current price using 2018 components. I know Apple gets much better prices than retail, but if I price an i5-7500, 8GB of RAM, and a 256GB blade SSD, I’m already over $400. That doesn’t include the motherboard or case, the cost to design, build, ship, and advertise. Yes, Apple has billions in cash, but they didn’t get there by making bad business moves.
Like everyone else, I’d love to see a new mini, but sadly current economics means it is either going to be poorly spec’ed or too expensive. I think this is the reason Apple is looking at an architecture transition—it’s the only way they believe they can deliver systems at low price points without them being crap. It’s why iPad is the cheapest Apple product.