I've done low level network programming for most of my career, and I've done a bit in the Linux network stack, as well as writing two partial userspace TCP stac

I've done low level network programming for most of my career, and I've done a bit in the Linux network stack, as well as writing two partial userspace TCP stacks for different applications, but both were for middlebox type appliances where the goal was primarily to replicate enough of what the TCP state machines would look like on both ends of the transaction to do make some decision (in one case for firewalling, and the other was doing analysis to try to make inferences about user behavior on one end of the connection when they could be pivoting through multiple devices in the network. It feels like networking isn't really a special case at all, just like most things the OS networking stacks are well understood, mature, and probably not worth trying to replace if you need a flexible general purpose networking stack that balances latency, throughput, correctness, security, memory and cpu efficiency, etc. But because they are trying to balance a lot of difference concerns across a wide swath of use-cases it's not hard to implement a better version if you have a very specific need.