At what point does it make sense to replace the CPU and OS with custom hardware and software? At this point the CPU is basically doing TCP state maintenance and

At what point does it make sense to replace the CPU and OS with custom hardware and software? At this point the CPU is basically doing TCP state maintenance and DMA supervision, but not much else, right? I totally get the cost, convenience, and supply chain risk-value in commodity stuff that you can just go out and buy, but once you're bound to a single network card, this advantage starts to go away, and it seems like you're fighting with the entire system topology when it comes to NUMA, no? Why not a "TCP file send accelerator" instead of a whole computer?