Why though, why should Nostr solve what Matrix protocol already did extensively, Nostr shouldn't reinvent private messages or rooms, ammend and incorporate what's already great out there.

The simplicity argument won't hold forever as you tackle hard problems.

Alternatively we can use a decentralized discoverable identity that abstract both Nostr and Matrix and much more, so users and clients use what the web already has.

Shameless plug etc pkarr.nuh.dev