Most upvoted comment
Apple, please make this cable.(r/apple)
You can buy a bus-powered 4-port hub for an order of magnitude less than that cable would cost.
Thunderbolt is external PCIe (plus miniDP). If you want to hook USB devices to Thunderbolt, you’d need to put a USB host controller (with a PCIe interface, and a thunderbolt transceiver) in it. And a 3-port USB hub (one port goes to the USB host chip).
Buy this for $2.50 instead.