I don't use Matrix a bunch so it might just be that I'm not as familiar and out of the loop. To me Keybase (despite all the drama) seems like the most isolated/pure example of a product that took the approach of username/password style accounts and applied it to application layer crypto to achieve secure messaging. Keybase later added all the network-y chat type features that make me think more of a product like Matrix. But if Matrix is good for 1:1 "chat up my contacts and groups thereof", then great. Matrix always seemed more like federated Discord or "crypto" IRC to me with the whole needing to join channels thing.