I always thought about npub as something like an IP address. You can't hide it if you want to communicate. It's part of a network packet. Sure, you can use a different one, but then all old connections stop working.

You can wrap it into a frame and use some "mac address" for local delivery. Then some relay/client need to understand this protocol.

so it's basically what we already have without nostr. routers are relays. ips are gateways and mac address is your npub

if it makes any sense