I can imagine that many cores starting to strain the memory subsystem in certain workloads.
As for a 16 core Ryzen VS threadripper, there's certainly room. They already differentiate the high core count Threadripper from EPYC by the IO. If you need a bunch of IO and memory Channels, and/or lower memory latency, you need an EPYC, but if you just want more cores, Threadripper is the way to go. I can see them saying the same thing for Ryzen vs Threadripper. If you need a bunch of cores, but are fine with just 2 memory channels and 20 PCIe lanes, then Ryzen is good enough, but if you plan on loading up on GPUs or gobs of memory as well, then a Threadripper is the choice.