But exactly that's the point. It can be handled transparently in a much easier way then the Ipv6 switch. (Because IPv6 is so much more different then just IPv4 + more addresses, and worse, many people don't realize it and treat it as IPv4 with more addresses which resulted in many problems).

For HTTP/2 at least, I think the main benefit in terms of performance applies to the "last hop", so you still get a more reliable experience even if the connection between the CDN/proxy and app server is http/1.1

True, through many web-sites don't have to care about the performance difference.

But for companies like CloudFlare or Googl HTTP/2 means less traffic overhead (multiplexing+header compression) and can save them a lot of bandwidth (aka. money) with that.

Exactly. AWS and GCP load balancers accept HTTP/2 but allow for those requests to be forward to backend instances as HTTP/1 because of this.

In fact AWS ALB does not even support HTTP/2 on the backends which is really annoying.