Ideally, I agree. In practice, until federation / decentralization is completely transparent to the end user (unless they choose otherwise), it’ll never be adopted at a large scale. IMO that’s one of the main obstacles of Lemmy, Mastodon, and others.
Signal is only relatively popular among the privacy-respecting options because setting it up is as easy as setting up WhatsApp. Just by adding a “choose your instance” step, you can cut your user base by an order of magnitude. And that’s not mentioning the quality of service, which is much more achievable on a centralized platform, whether that’s in terms of feature parity, uptime, bug fixes, or cross-platform support.