Possible better peer-to-peer protocols for Bitcoin?
The network used today isn't actually the same as the original design, which did not have an inventory system at all, every transaction and every block was sent to every peer indiscriminately. Compact Blocks was also added to remove redundancy in block transmission and reduce latency, as blocks predominantly contain transactions which have already been forwarded around the network previously. Fee based filtering was added to allow nodes to request that peers to not send transactions which will be rejected regardless, which saves bandwidth.

The P2P protocol, while not ideal, is relatively functional in its current state. It is an unusual set of requirements (flooding, censorship resistance, sybil avoidance) which doesn't have much simularity to many other systems.