Of course, although you may be able to go very much faster, you also now took on all this extra responsibility, and so complexity, and so likely bugs.

Chances are the weird corner case where the TCP implementation blows up was seen in a dozen other Linux applications and fixed, but the same case inside your entirely userspace QUIC implementation gets to explode while the boss is doing the live $100M pitch demo. Awkward.

Code that is competing with Linux for performance is likely to be highly concurrent (a big risk factor for bugs) and doing fancy gymnastics with memory (another risk factor) so we're not talking about a landscape where bugs are rare or one where they're very easy to track down.

I believe this sort of thing is one reason Linux is putting more emphasis on safety. As usual for HN this bug already has people saying both "Rust wouldn't catch this" and "actually Rust would catch this", but even if you're quite sure Rust isn't the answer a safer Linux by whatever means makes this argument better for Linux than it is today.