LOL I love this stuff, I mean.. I've seen this a few times here. "You don't know what you want, what you really want is something else!
My message might have come across that way, but that doesn't change the facts. There simply is no large group of Apple consumers who want a mid-tower. You are just wrong about that. Sorry to be the one to tell you.
It's also untrue that many people *ever* upgrade their computer beyond a stick of memory and (maybe) a new video card. So my comment about what people "think" they want vs. what they actually want or need is pretty much right on the mark. This has been pointed out in the media many times over by people much more knowledgeable than me.
... a lot of potential switchers I talk to would definitely buy a mid-range if it was there.
Well, here you are
agreeing with me about "switchers" being the primary group interested in the mid-tower. Let me say you are absolutely right about me being right about that.
Also gamers are real Mac users too.
Here I think your smoking something pretty weird. If there ever was a demographic that is almost the complete
opposite of the average Mac user it is the average "gamer." X-Box goons are noteworthy for their Apple bashing and their fierce unreasoning loyalty to Microsoft if nothing else, and the majority of PS3 users are Japanese. Japan is one of the only markets that Apple has difficulty in and has one of the highest Windows penetrations of any market AFAIK.
The past sales of similar machines don't mean squat - if Apple did it right they'd sell like crazy.
Okay now I am wishing I read this bit before I started replying.

This indicates to me that you have no concept of markets, products or sales at all.
One of the reasons a mid-tower is not a good idea is that the sales of the all-in-ones that fill this niche is through the roof. Making an argument that a product they are literally shoveling out the door should be replaced by a different one and claiming that the furious sales of ... well the machine that is selling, is irrelevant, is just nuts.
You are ignoring the obvious. Apple is already "doing it right" with the iMac and the sales prove it. A more important market segment to measure if you want to support your argument is those people who *did* want a mini-tower, that were then convinced to buy an iMac and were subsequently so disappointed that they *still* wish for a mini-tower and that they could take the iMac back. I would bet this is a very tiny group of people. Prove me wrong.
And the argument that it'd cause confusion is offensive really, there's already more variety in the iPod line than the Mac desktop one...
Wrong again methinks.
With few exceptions, Apple always has three product lines in each segment, a mini/nano, a mid-range, and a pro. iPod is the same as the desktops and portables in this regard, although I will agree with you that there has been a lot more variety in the iPod lines lately. Apple always strives for that three model segment though and one would expect that once the transition to flash is made they will return to that formula.
You are wrong about the ACD's also, they are almost the best monitors you can get, and for what you get they are good value. I have occasion to see dozens of different new (non-Apple) monitors a year and test them out and almost without exception they have lower resolution, poorer colour management and inferior construction. Even the good ones are generally much poorer than the equivalent Apple part, but the majority of users that would be happy with a mid-tower would also be happy with some Ben-Q POS monitor and not even see a difference. These consumers are not Apple's market.
Another really good sales-based reason for *not* doing a mid-tower is that it would take away from the sales of the existing segments.
If the iMac is selling like hot-cakes with a customer satisfaction rate that is through the roof (it is), and if the MacPro is doing similar gangbusters (it is), why would you introduce a new model that would eat into the sales of both? It adds to manufacturing costs, adds to support costs, and would have a similar margin to every other mid-tower out there (razor-thin). Apple would be destroying or throwing away at least *some* portion of those iMac and MacPro sales at a very *high* margin, and replacing it with sales of a low-margin product. There simply is no upside for Apple here.
Now since Apple is one of the few companies that thinks of the consumer first, they *might* want to do that just because "so many people want it" as you assert. But just as what I said when I started this post, I don't think you are right that this group really exists. If it does exist, I don't think it's large enough for Apple to throw away money on.