It's not implementable by local devices: your LAN is much faster than your WAN link, in most cases. Your router/modem does the packet dropping. But only your router/modem knows how much unused throughput is available at any moment, so rsync can't know how much it can use. Sure, rsync can look at TCP packet retransmits, but that will still cause other streams to experience an (assuming a lot of things) "equal" amount of drops too.

So, you get a lot of complexity for not a lot of benefit. However, Fuchsia is destined for mobile devices, which may not have the LAN-WAN bottleneck, so this could see some benefit.