Why should we be concerned that Signal isn't decentralised?
I like the fediverse being decentralised because everyone doesn't have to play by the same rules. But it's different for Signal because it's not a public forum. They basically allow everything there that isn't spam or illegal IIRC. And it's end to end encrypted so they can't see anything anyway.
Do you mean so that there isn't one organization that has access to everyone's metadata? That makes sense actually now I come to think of it.