You have drifted into hardware.
Why separate the two? We are talking about using a bit of hardware as a professional tool.
Magma chassis is limited and expensive. It is cheaper to buy a Mac Pro and stuff it with cards, especially if you want to run HD 4 or 5, which most professional studios are using. A Magma ExpressBox 4 is over $2,000 by itself and does not give the best performance. Not ideal. The ExpressCard solution is severely limited in most real applications. It is a expensive square peg in a round hole where the expandability of a Mac Pro is suited out of the box.
There is nothing that precludes Avid from taking the older PCI-e based design for the HD model and wrapping it in a TB container box and shipping that. Refactoring the HDX design for some slightly more limited space/power requirements would probalby only take a little bit of work ( so them not qualifying expansion boxes is fine. With some work AVID could ship their own expansion box and pocket the profits.... they certainly are hard up for cash and revenues so not exactly a risky move on their part. )
There's nothing to stop them, except the fact that the install base is more likely to move to Windows rather than buy an iMac or a Mac Mini.
Avid did have an expansion box in the past and stopped selling it because there were very few sales and supporting the product was a hassle.
Again dragging in hardware. Most often legacy hardware with non mainstream adapters and/or components.
Monitors being legacy hardware? If you want a three-display system with fast response time, which Mac in the current lineup makes the most sense? Does someone editing video want to use the screen on the current production iMac?
yet again .... more disk capacity (more hardware) as being the blocker.
It has nothing to do with capacity, it has to do with throughput to that capacity. Buy an iMac or a Mac Mini and you can now use Thunderbolt, which is nice, but rather expensive. A Mac Pro is actually more economical here. I have four drives in my Mac Pro; system/boot, audio, samples and backup. If I had to use FireWire it would be limiting (and is on my MacBook Pro). Start adding up the costs of outfitting an iMac with the proper Thunderbolt storage versus buying a Mac Pro and slapping in a couple of disks, and the Mac Pro comes out ahead on performance and price.
As I said. There are some workloads that will drag in some hardware subsets that present a problem. In the vast majority of cases are not all of the contexts where the software is used though.
I never said most people need a Mac Pro, but your statement regarding "flag wrapping" diminishes the niche that does exist and will need to be served. A Mac Pro in a studio these days is as much about marketing as it is about preference. It is definitely not about a
need these days given the performance of Windows. The same can be said with Pro Tools HD.
I am not worried about the future. I hope Apple serves it, but they are not going to sell me on a machine with an integrated screen and non-user serviceable drives. I will happily move to Windows. It's just a tool and I am not a cheerleader.