Except there are already several such implementations and nobody is going to deprecate older HTTP versions.
Chrome has an overwhelming portion of the web browser installed base. It's full of dark patterns and scary warnings about "dangerous" things like unencrypted HTTP. I don't have a lot of faith that they're not going to disable older HTTP versions at some point.
And that is one of the key "benefits" of HTTP/3 if you're a Google-scale company.
There are many implementations of QUIC already, its quite diverse...