The mistake they did was to assume only one TCP socket should be used; the TCP has it's own head-of-line limitations just like HTTP/1.1 has if you limit the number of sockets (HTTP/1.1 had 2 sockets allowed per client, but Chrome doesn't care...) it's easily solvable by using more sockets but then you get into concurrency problems between the sockets.

That said if you, like SSE on HTTP/1.1; use 2 sockets per client (breaking the RFC, one for upstream and one for downstream) you are golden but then why use HTTP/2 in the first place?

HTTP/2 creates more problems than solutions and so does HTTP/3 unfortunately until their protocol fossilizes which is the real feature of a protocol, to become stable so everyone can rely on things working.

In that sense HTTP/1.1 is THE protocol of human civilization until the end of times; together with SMTP (the oldest protocol of the bunch) and DNS (which is centralized and should be replaced btw).