The pro machines never sold well. The Mac Pro had laughable sales,a s does the Mac Mini. Apple really kept them along because of the small by very vocal community who can be guaranteed to buy a few thousand units.
The purpose of pro machines isn't to sell well. The purpose of pro machines is bragging rights—specifically, being able to say that you build machines that are some of the best on the market, and being able to say that people do amazing things with your machines. But sure, if you want OS X to turn into a passive media consumption platform like iOS, keep dumbing down the hardware. Pro users will start using other platforms to do real, creative work, and eventually OS X will wither and die.