What makes X and Instagram work so well as public boards is that everyone on the planet uses them, so even though it's 90% trash your potential for meeting cool people is basically limitless.

I don't think that will ever be replicated by Nostr or anything else like it.

Messenger apps are different. SimpleX or whatever could easily replace WhatsApp/Discord/Telegram/SMS/whatever else if they had just enough reputation for people to download them, even if it's not totally mainstream.