All these arguments are valid, however, there are flaws in your reasoning. Your hope for an upgradeable mid level tower are simply not something Apple will do in the near future, or possibly ever. While RAM and HD upgradeability is a feature of damn near every product line Apple offers, the video card upgradeability is restricted to a select few products. And considering the delay in the delivery of the Nvidia 8800 to the Mac Pro's it simply is not all that important to Apple.
Your dreams of having a Mac gaming machine will not happen untill there is a stable games market for the Mac. Considering that the PC and the console games market completely dwarf the Mac game market, it seems unlikely that there will be a major shift to Mac development any time soon. There simply arent that many profits to be had here.
Additionally, your argument that a mid range tower will be better for corporate users holds no water. For a lot of corporate users a Mac Mini or an iMac are a perfect computer. The average user uses it mostly to do email, and various projects. For whatever company employer that needs the extra oomph, the Mac Pro is a far better option than the minimally upgradeable mid level tower.
Your desire for a mid level tower also disregards the fact that Apple, as a company interested in making a profit off their hardware sales, would introduce a machine that clashes with their consumer and pro lines, which is somethign a mid level tower you are dreaming of do.
For any profesional user, a Mac Pro is neither too big or too bulky, because it packs everything they need and for every home user, the iMac is perfect because it packs everything they need and more in a convenient package. The mid level tower you desire does neither. It doesnt have the expandability of the Mac Pro, nor the small size or convenience of an iMac...