To turn your own question around: Why bother trying to optimize a stack that contains a lot of stuff you don't strictly need? The talk doesn't deal with general-purpose servers that perform multiple roles; it says "if you have a singular role, here is how you hyper-optimize to support that role at a huge scale."
You only have one specialized need at the very beginning. Then you start seeing the need for "just another little feature". And soon you will start replicating big parts of the network stack. Call it Greenspun's tenth rule for the network, if you want.
Also, most of the time, what you "just need" happens to be also the common need of many other users out there. Joining forces to fix a single code path is a much better investment than redoing things in userland for the sake of it.