Flow control can definitely be useful when streaming audio/video files. A browser only needs a few megabytes of buffer to display 1080p content properly. Everyday example: user starts watching a youtube video, gets interrupted and pauses it, then closes the browser because they got carried away. The entire video will be downloaded and put in the cache where only a few megabytes were needed/watched. That's quite a waste of bandwidth at 1080p.

Multiplexing allows you to have the same level of concurrency as you currently have with domain sharding without requiring multiple TCP connections (and TLS contexts). Starting and getting a TCP connection up to speed takes a while, especially on congested links (packet loss, spurious retransmits, slow starts, etc.), and TLS just makes the problem worse by requiring multiple back and forth to setup encryption. It also takes quite a bit of memory on servers to maintain hundreds of thousands of TCP socket and TLS states, and a lot of CPU to set up TLS contexts (Diffie Hellman can be quite expensive CPU wise). Then, there are flow-based routers, load balancers, stateful firewalls and other stateful network equipments. We'll get greater performance out of them by using less concurrent connections.

I think moving to a binary protocol and reducing the number of TCP/TLS connections is a very good thing, long overdue IMHO.

EDIT: typos :)