Not as simple as that as many did ditch WahtsApp for Meta's documented privacy violations, and their ongoing T&C which passes the WhatsApp metadata upstream to Meta and others. A lot of people also only use one messenger, and right now nothing connects them together yet. So I have masses of family and friends that only use WhatsApp, and I now only have SMS contact with them. About 8% to 10% do have multiple messengers so I see some on Signal and Telegram.
The last thing the world needs, is for WhatsApp to become the default dominant standard. That is a company that can be least trusted out of everyone worldwide, based on their history. With the app installed, the metadata includes constant location, usage, contacts, messages to who, etc.