[noway421]

What if your browser receives Alt-Svc header and switches to http/3 on one network (say mobile data), but then you switch to a restrictive WiFi that has UDP disabled. All without restarting your browser/within one "session" of your http client. Wouldn't you start having connectivity issues that would be hard to troubleshoot? In that scenario, having http3 disabled is beneficial.

[fridek]

Changing network interfaces breaks connections and causes a new handshake. Browser session works at a different layer and doesn't prevent that.

QUIC actually lets you migrate between connections (because the packets are identified by a connection ID in each UDP packet rather than a 5-tuple). Clients will typically re-test a connection occasionally and downgrade as needed for this to work.