You don't have to take over the hardware to do userspace TCP. You only need the ability to put raw datagrams on the wire (Linux RAW socket, for example). You _can_ take over the device with something like DPDK but that's not necessary. As you correctly say, QUIC uses existing OS-level facilities to communicate over UDP, so there is no reason that several processes on a single host cannot use application-level QUIC stacks concurrently.