Yes, QuickSync has a nice hardware accelerated H.264/HEVC(H.265)/VP9 encoders. Its quality is ok for streaming online in realtime but most of professionals use software encoders for high-end quality. So you still need a high number of cores/threads for tasks as video encoding, rendering, compiling etc. Quad-core won't do.
In other words, QuickSync isn't replacement for extra cores.
Low number of cores (4 instead of 6, 8) and only mobile line (TDP only up to ~28W) speak of low yields of Intel's 10++nm. It's not part of design but rather a mitigation of a problem.