Far from it. It's quite likely going to be over 5 years if not a decade before it would be possible to run a pure-HTTP3 service without risking connectivity problems

The problem is similar to the IPv6 transition, except thanks to the browser monopolies, it's possible at least for network providers to quickly feel significant pressure to fix their networks. But there will always be some networks that will never be fixed

edit: for those inexplicably downvoting this, please pay attention to the parent comment's question, and the Internet's long chequered history of adopting new protocols in any setting. TCP port 443 isn't going to magically disappear overnight, or indeed any time soon. This is evidently true because it has been true for all prior transitions. Mail still flows to many places unencrypted despite the standardization of STARTTLS 21 years ago. The long tail has only gotten much longer in those intervening 21 years.