If there is still one company pushing their own strongly opinionated client, how does having competing options available to power users detract from that? Like I'm not saying Signal-the-client should go away, rather that Signal-the-company who runs Signal-the-server could be accepting of third party clients, to the point where Signal was easily usable from F-Droid and whatnot. How would that undermine the mass market option?

They do accept third party clients. They don't like it, but they don't have a choice, because they don't use device attestation.