I hate it when people ask for midrange towers. It's not going to happen.
If Apple made such a machine they'd probably price it higher than the iMac line since it would be more upgradeable and therefore last longer. Also, while there's demand on these forums for it, I highly doubt the real world demand for such a machine is high. These two things would probably price it just under the base Mac Pro which would cannibalize on sales of the low end Mac Pro.
Why tool a factory for computer that's not going to sell? It's not going to happen.
The Macs that exist are not niche products because of their specs. Most people grow up on windows machines and they continue to buy what they know. This is why Apple started Boot Camp. As an example, there's a Mac Pro in most production houses in the world. It's not a niche product. Most design shops have a lot of iMacs. They're not niche products. A lot of producers and students have MacBook Pros and a lot of students have MacBooks. They're not niche products.
People who have midrange towers are usually hobbyists that prefer to assemble their own systems anyway. To beat a dead horse, it's not going to happen.