Isn't 8 GB per core still reasonable? Especially if you can share indices and buffer caches across cores, for example. There must still be diminishing returns for caching.
Depends on your dataset. Systems like Databases perform much, much better the less you have to touch a storage controller. One of the things that's going into Intel's 2017/2018 platform is the support for 6 channels of memory, and not all of that needs to be DRAM. There's a new standard, NVDIMM, that lets you use non-volatile memory as RAM, and take advantage of its much higher densities. So, if you have things like a database, you can map a much larger dataset into actual memory, instead of needing to just have the indexes in there.
I do a load of work with high throughput genetics sequencing. One of the programs that processes the data for a sample is single-threaded, but requires about 40GB of RAM. I currently have a server with 384GB RAM and 64 CPU threads, so 6GB/thread. RAM capacity is a definite bottleneck, since I can only run 9 at a time, instead of 64.
It all depends.
"reasonable" is entirely context-dependent. Some workloads are CPU or network bound. Others are memory bound.