Ironically, one of the proponent documents of QUIC and HTTP/3 (https://http3-explained.haxx.se/en/the-protocol/feature-udp) actually calls out how QUIC doesn't solve this problem either. Middleware boxes are blocking or deprioritizing UDP packets already, so UDP based 4.5 protocols can't solve the very problem they claim to.
Circumventing badly behaving middleware is obviously not actually possible. You can't account for every protocol-breaking/limiting decision every middleware vendor has ever made, and the "they'll come around" argument is a better argument for NOT using QUIC.
It has to do with the distinction that middleware boxes make between layer 4 and layer 5. NATs and firewalls and assorted traffic optimizers tend to vomit if you pass anything over IP that isn't TCP, UDP, or ICMP.
UDP is, by luck and design, a very thin shim on top of which a "layer 4.5" protocol is very easy to implement. Whether this layering is done in the kernel or in a userspace library is an implementation detail, and one that will probably be obscured even from many userspace programs.