without a scheme to pay for it it's gonna be bogged down with spam, and no, PoW isn't gonna solve the problem forever (tor project has kicked the can with their use for rendezvous routing) because eventually someone will find it valuable enough to overwhelm average user hashpower to push spam without any recourse for users or on the network

i've worked on a huge codebase, over 20k lines of code for doing this and it's not a simple task to implement, and we still haven't got clients supporting NIP-42 yet

also, aside from everything else, this is outside the domain of relaying in an architectural sense, you are essentially proposing to turn relays into a network transport