When talking about messaging apps, you’ve to use the one that’s easiest to use even for the most noob tech person.
Signal uses phone number, because that’s how people generally communicate. It’s easy to use, setup. No privacy nightmares compared to WhatsApp, or security nightmares compared to Telegram (where E2EE is not even on by default). It’s open source, regularly audited and can be used on any device (no more proprietary green bubble nonsense). There’s still a market for Threema and Matrix, etc. It just never will be mainstream.
True, I’d rather support Firefox as a browser and an organisation. Edge for pages that work best with Chromium engine