Http/1 Keep-Alive was one the earliest protocol performance improvement. It meant the TCP connection could be reused for the next resource, avoiding the overhead if closing and reopening a new connection for every request to the same server.
You can see http/2 multiplexing as a natural evolution to the keep alive. Instead of doing every request sequentially, just send everything you need at once and let the server figure out in which order to process everything.
Concurrent TCP connections do have an impact on all stateful firewalls in your path, stateful load balancers and of course the server itself. A client that opens dozens of simultaneous TCP to request all the resources on a page all at once would be frowned upon, and quickly hit rate-limits and anti-abuse protections. You can call it ossification of protocols but that is the situation even google could not workaround when specifying http/2 and then quic.