But why they don’t want to is the salient issue. Yes, they could fill the gap easily. But could they do so at a profit or is it a loser?
It’s one thing to have a niche Mac Pro, with an ASP of $10k, where there’s $3-4k gross profit. And those types of customers will buy higher-end configs from Apple, at Apple’s prices. That’s a viable product.
The product you want is more for a DIY’er who will buy a $2k machine from Apple, then order some DIMMs, a GPU and an SSD from Newegg or Amazon and “have at it”, as you say. I get why that’s attractive to people like me you and me who would benefit, but realistically, how does that benefit Apple? A viable product has to make sense for both the customer and the company.
A larger mini becomes problematic once you start talking about PCIe slots. Now you need a 300+ Watt power supply and you’ve got to cool the thing. It’ll be much larger and louder, and what’s it all for? Those who want a more powerful GPU, mostly. That edge case is addressed by an eGPU, albeit with some compromises.
My overall point is, this isn’t just a case of Apple “just not wanting to” out of spite or because they don’t care about the customers who want this. Apple is a business, and they make business decisions. Sure they like to do good things and make their customers happy, but they’re still a profit-making enterprise.
Making a relatively small number of great products best-suited for the most customers is Apple’s calling card, and that’s inherently in conflict with being able to give all customers their ideal product. iPhone SE, xServe, AirPort, Time Capsule, Aperture, etc. fans feel your pain