I understand you are writing a server. To be fair, not all compute loads are embarrassingly parallel. For those tasks single thread performance - especially from the point of view of memory bandwidth - is critical.

Single core is not 'past'. 1. CPU:s are not getting that much faster every generation (one could argue that this means parallellization is critical, but...) 2. The number of cores on the average desktop is not that much. One of the reasons I think is that in many cases the individual tasks that must be performed sequentially in a typical desktop program are so tiny or hard to split that that the platform overhead and complexity of multiple threads cause the benefits from actually utilizing multiple threads to be a bit harder to reach than just using a language with a nice parallellization story. By 'benefits' I mean making the program actually faster for the user.

Of course, I'm talking about Amdahl's law https://en.m.wikipedia.org/wiki/Amdahl%27s_law