The client will kill the connection if it has the file cached, sooooo, not much.
With the war on mitm, it's really hard to set up something that scales traffic in this way - even if the actual data requested by clients could readily scale.
I know it's a trade-off between security and features - but it still makes me sad.
That tells the client what the server wants to send.
The client can respond with a an RST_STREAM frame https://http2.github.io/http2-spec/#RST_STREAM Again, that's a single frame.
By design it's meant to be extremely small and quick even on high latency, and/or low bandwidth connections.