You can build a custom L7 on top of anything, really. I think my favorite was tcp/ip over printers and webcams.
The question is what does QUIC get you that UDP alone does not? I don't know the answer to that. Is it because firewalls understand it better than native wireguard over UDP packets?
The hope with QUIC is encrypted tunnels that look and smell like standard web traffic are probably first in the list of any allowed traffic tunneling methods. It works (surprisingly) a lot more often than hoping an adversarial network/security admin doesn't block known VPN protocols (even when they are put on 443). It also doesn't hurt that "normal" users (unknowingly) try to generate this traffic, so opening a QUIC connection on 443 and getting a failure makes you look like "every other user with a browser" instead of "an interesting note in the log".
I.e. the advantage here is any% + QUIC%, where QUIC% is the additional chances of getting through by looking and smelling like actual web traffic, not a promise of 100%.
You really don't want reliable transport as a feature of the tunnel unless you are _intimately_ familiar with what all of the tunneled traffic is already doing for reliable transport.
The net result of two reliable transports which are unaware of each other is awful.
Mostly because WireGuard (intentionally) didn't bother with obfuscation https://www.wireguard.com/known-limitations/
> WireGuard does not focus on obfuscation. Obfuscation, rather, should happen at a layer above WireGuard, with WireGuard focused on providing solid crypto with a simple implementation. It is quite possible to plug in various forms of obfuscation, however.
This comment https://news.ycombinator.com/item?id=45562302 goes into a practical example of QUIC being that "layer above WireGuard" which gets plugged in. Once you have that, one may naturally wonder "why not also have an alternative tunnelling protocol with <the additional things built into QUIC originally listed> without the need to also layer Wireguard under it?".
Many design decisions are in direct opposition to Wireguard's design. E.g. Wireguard (intentionally) has no AES and no user selectable ciphers (both intentionally), QUIC does. Wireguard has no obfuscation built in, QUIC does (+ the happy fact when you obfuscate traffic by using it then it looks like standard web traffic). Wireguard doesn't support custom authentication schemes, QUIC does. Both are a reasonable tunneling protocol design, just with different goals.