I don't think it will entirely replace platforms like SN, but who knows. Protocols like email are comparably versitile (I know there are key differences but bear with me) yet they are not the only method we use for p2p style communication. Same thing is likely to happen with nostr, especially since other fediverse protocols have been gaining traction. Likely, we'll have a variety of platforms and protocols with varied degrees of decentralization. Such a world leaves plenty of room for standalone sites like SN.