Unlike IPv6, QUIC requires (almost?) no buy-in from network operators/ISPs.
Which is pretty much the entire point of QUIC, since updating gazillions of network appliances to support newer versions of TCP is rather hard and will take decades, if not longer. With QUIC, that's no longer needed.
Handy but Germany is the only country that has 50% IPv6. I am still calling it's not going to be rolled out any time.
After all the equipment investment and adding support to all the software, people are going to say "just forget it" and go back to v4-only and piling CGNAT style hacks on top of each other in an endless tower of babel? Possible, but would be quite a dystopia.
IPv6 transition mechanisms were designed for a long coexistence of both v4 & v6, not that different from QUIC vs HTTP :)