Matrix is strongly persistent, thin client; Signal is ephemeral, thin server. They cannot replace each other.

It doesn't really matter, because you can replace SMS with linearized Matrix. Signal can't, because it's centralized

Signal the protocol can be and is implemented by third parties. Signal the platform, is different. Keep in mind Signal's only centralised part is thin server that acts as a relay. Matrix is a thin client protocol that relies on server to enforce soundness rules.

IDK what's your point. A encryption protocol isn't enough to replace SMS. Linearized Matrix is, and uses the Signal Protocol, though MLS probably would be a more future proof solution