I think it wasn't just the growing core counts on the mainstream platform, but also the death of multi-GPU gaming and steady growth in DDR4 speeds and NVMe SSD

I think it wasn't just the growing core counts on the mainstream platform, but also the death of multi-GPU gaming and steady growth in DDR4 speeds and NVMe SSD capacity and performance—all of which combined to eliminate the need for a larger socket with all the extra IO.