This shouldn't be all that surprising. You don't normally see CPUs (or GPUs for that matter) with 3/4 of the cores disabled. Even 1/2 is pushing it. By the time Summit Ridge is released, Polaris 10 and 11 will have been on retail sale for about six months, so it isn't even a bleeding edge node for AMD.
Intel hasn't made a quad-core die with no iGPU for a long time. Apparently, there just isn't the market for it. AMD, with its much smaller market share and limited R&D resources, would be foolish to create such a niche product. Summit Ridge, with 8 cores, is meant for servers, workstations, and high-end desktops, where an iGPU is not needed. Raven Ridge will be the Zen APU coming in 2017, and it will probably have 4 cores and be aimed at more mainstream audiences.
Summit Ridge won't be a particularly large chip - maybe not much bigger than Polaris 11. AMD should be able to release the cut-down 6-core versions at a very competitive price while still maintaining a decent profit margin. I think it's likely we will see the full 8-core chips at pricing comparable to Intel's i7 series, and the 6-core chips at pricing comparable to i5.