Earlier quoted context omitted.
Why can't QUIC be implemented in the Kernel? Just because something can be implemented in user space does not mean that it has to.
Anything can be implemented in a kernel. But unless it's natively supported in other kernels, networks, and systems, that just means that one kernel is easier to work with. The big idea behind TCP/IP is that it's a stack which you can use anywhere and everywhere. Not just any network, but any host. To do that, you need it to be portable and ubiquitous. And there is no protocol above OSI layer 4 that is ubiquitous. By…
For what its worth, Windows 3.x did not come with a TCP/IP stack installed; it was an optional extra. (In the days before Windows 3.x you had to pay for the TCP/IP stack from your network card vendor as an optional extra.)
It wasn’t until Win95 that the OS came bundled with TCP/IP by default, and you could still install other networking protocols like Novel Netware or AppleTalk.
TCP/IP didn’t become the default stack until late into the ‘90s and the “everywhere by default” wasn’t really true until just before the turn of the millennium.
So while QUIC might be in the “early adopters” space for now, everything started that way once. I have no doubt that by 2030 we will see QUIC as a kernel feature in many major operating systems.