I expect HTTP/2 to disappear completely, to the point of browsers removing support for it probably even within a decade. Everything that works over HTTP/2 should work over HTTP/1.1, even if at a slight cost, and HTTP/3 should be uniformly superior to HTTP/2, except in those rare cases of UDP-blocking firewalls, which situation I expect to improve over time. Given the complexity and maintenance burden of the protocol and more importantly the upgrade mechanism, I think browsers will be happy to remove HTTP/2 in favour of the old reliable HTTP/1.1 (which is certainly not going anywhere) and HTTP/3 once they see very little using it any more, and absolutely nothing needing to use it.