Ok, and would it not be possible to just add those features to a Mac Mini and any slots/bays in an accessory that can be added to a stack?
Along with 12 or more cores of CPU? And four bays of external expansion? And eight ram slots? No, that wouldn't be possible. Some people actually need the power the MP provides, even the top of the line iMac isn't enough.
You're absolutely right that Apple doesn't want to put much effort into niche markets. But coming up with some sort of modular thing would be vastly more R&D and still be selling to a niche market.
Really the only downside to a niche market is the development costs compared to the money they bring in. They're never going to have big growth in that part of the market. But the good news is they can update the Mac Pro with relatively low cost and manpower required. Apple does redesigns and they do spec bumps, and what the MP needs is more of the latter.
I imagine Jony Ive will have had great input with the Logic X GUI
Cool idea, but I see a number of problems with that. Logic 10 has probably been in development for years now, and Ive hasn't been in that position that long. Ive isn't a music guy - it would be great if he gave some tips on aesthetics, but I don't want a guy who's not a musician designing the interface. And most importantly, Ive has way too much on his plate to have time to be involved with Logic. iOS7, OSX 10.9, along with all the bundled apps on both platforms. If he has any free time beyond that, I would think he'd be looking at things like iLife and iWork. Hopefully Apple is putting more resources into Logic, but having Ive directly involved seems really unlikely.
I understand that a Mac Pro offers more flexibility but Apple can deal with that on the Mac Mini side.
They're not going to get the max cpu power (12 cores, probably 16 or more in the next generation) of the MP into a mini form factor. There's a reason they stick with mobile cpus, if they go to the higher power consumption and heat of desktops (and go to dual chip), it's back to a form factor more like the MP than the mini.