Isn't the point of QUIC to offer high performance and flexibility at the same time? For these requirements, a one-size-fits-all API is rarely the way to go, so

Isn't the point of QUIC to offer high performance and flexibility at the same time? For these requirements, a one-size-fits-all API is rarely the way to go, so individual user-space implementations make sense. Compare this to file IO: For, many programs, the open/read/write/close FD API is sufficient, but if you require more throughput or control, its better to use a lower-level kernel interface and implement the missing functionality in user-space, tailored to your particular needs.