There are a billion timers inside the kernel, not all of which can be changed. Some of them are #defined even.
In these days when machines are very large and always have several applications running, having an external network stack in the kernel violates the end-to-end principle. All of the policy about congestion, retrying, pacing, shaping, and flow control belong inside the application.
Can you point me to an example of a timer in the kernel that is not settable/tunable that should be? My experience in looking at such things suggests that most of the #defined bits are because RFCs define the protocol that way.
As for network stack per application: you're more than welcome to so so a myriad of ways - linux provides many different ways to pull raw IP, or raw ethernet into userspace (e.g. xdp, tun/tap devices, dpdk, and so on). It's not like you're being forced to use the kernel stack from lack of supported alternatives.
In these days when machines are very large and always have several applications running, having an external network stack in the kernel violates the end-to-end principle. All of the policy about congestion, retrying, pacing, shaping, and flow control belong inside the application.
https://en.wikipedia.org/wiki/End-to-end_principle