I must've seen this suggested at least a half-dozen times on this forum. The problem is that many Mac Pro users don't want a Mac mini Pro, they want an expandable full-size tower with dual user-upgradable Xeon CPUs, multiple user-swappable desktop PCIe graphics cards, desktop ECC RAM, etc. All of this requires a much larger form factor, and a much higher price tag than can reasonably still be called a Mac mini (or Mac mini Pro).
A pro-focused Mac mini would appeal to a certain audience, especially with a sub-$1,000 starting price, but it shouldn't become a replacement for the Mac Pro.