Apple could easily make a Mac with i5-i7 CPU and latest-bestest GPU in the Mac Pro enclosure
...not so sure about that - the thermal design of the nMP was very, very dependent on that triangular core spreading the load
three ways between a CPU and dual GPUs, with the GPUs doing a lot of "GPU computing" work via OpenCL. The nMP is as a dead-end design built around one specific configuration.
No, the hole in Apple's range is still a desktop system with an i7 and a couple of PCIe slots and a choice of GPUs that is compact, but not so much as to cripple it thermally. Call it the "Mac Truck"

Trouble is, it is never going to be a money-spinner in itself, but would be an indirect strategic asset by keeping the Mac platform popular with enthusiasts and developers (you know, the people who persuade family, friends, colleagues, bosses etc. to buy Macs and ensure that there is a good range of software and support available - either by writing it or providing a market for it - and don't even charge a commission).
10 years ago, the mini tower was still the mainstay of personal computing, and a Mac mini tower might have cannibalised higher margin iMac and laptop sales - these days, I'm not so sure.
It would be
absolutely brilliant if the new Mac Pro range started with an i7/desktop GPU combo at an affordable price - especially with these new i7s - but I expect its going to be a Xeon/ECC thing starting at a minimum of $4000 that
might be good value if you're a video pro who can put a price tag on every 10 minutes of editing time and every glitch in a 48 hour render job that might have been prevented by ECC...