On day zero? Sure. But then good networks will slowly enable UDP/443 anyway, and have good cover because it's The Web, and eventually it'll be enabled on more than 90% of networks.
Then a few non-browser applications that use QUIC/HTTPS will decide that 90% is good enough and start to not support TCP anymore, causing trouble for the holdouts and getting it enabled on 95% of networks. Then even more applications will do that.
At some point the browsers can put up a warning when fallback happens, telling the user that their network is degrading performance.
Eventually the number of networks that don't support it is small enough that nobody supports fallback anymore.
> How so? how is TCP 80/443 and UDP 80/443 harder to traffic shape than TCP 80/443 alone?
They can't mess with window sizes and such because they're inside the encrypted payload.
Modern TLS agrees the sub-protocol to use (in this case h2 = HTTP/2) early with ALPN. If that ALPN sub-protocol isn't available that same connection just becomes HTTP/1.1 (over TLS) instead.
So there's no reason to fall all the way to HTTP/1.1 without asking if HTTP/2 is available.
Doubt it. I suspect that browsers have some sort of happy eyeballs algorithm for determining whether to use http/3 specifically because some networks don't handle it well. In those cases it'll fall back to http 1.1.
>Now all those traffic shaping middle-boxes are worthless
How so? how is TCP 80/443 and UDP 80/443 harder to traffic shape than TCP 80/443 alone?
> and your ISP can no longer spy on your requests to gather marketing data about you.
That's more encryption (ie. https) than switching to http/3. Also, encryption is already mandatory for http/2 (for most browsers).