This is a common problem for any peer to peer protocol.

Doesn't really strike me as a problem though, it just means the client has responsibility of opening a port. Considering the complication of adding 1000s of hole punching strategies in the protocol, it's a fine trade-off.

Not really. Bitcoind and geth work just fine behind NAT. They have no hole punching strategies in the protocol at all.

Hu yes they do. Last I checked they both had NAT traversal through UPnP. I remember that years ago bitcoin even had peer discovery through IRC channels in case UPnP was disabled.

1) UPnP isn't part of the p2p protocol

and

2) even in the case where UPnP fails and there is no inbound connectivity, bitcoind works and syncs fine with only outbound connections

Solana won't even sync behind nat.