I'm not the right guy to answer this but there's a debate about that https://github.com/nostr-protocol/nips/pull/1206

And If I understand the concept it's not everything on the client side. Relays also have a specific role in the private messaging and users need to have rhem set up.

Can nostr:nprofile1qqsyvrp9u6p0mfur9dfdru3d853tx9mdjuhkphxuxgfwmryja7zsvhqppemhxue69uhkummn9ekx7mp0qythwumn8ghj7anfw3hhytnwdaehgu339e3k7mf0qyghwumn8ghj7mn0wd68ytnhd9hx2tch2deau say more about if Nostr is ready for private messaging?