the poor countries use WhatsApp mostly due the fact that sms was costly.
so you're tryna to monetize against businesses serving poorer users. yes - they maybe more in number but margins are razor thin.
the richer countries where margins are higher - sms, email, etc are cheaper & permission less. so eventually most providers most settling on email & sms/rcs.
But NB even where messaging apps are defacto norms, most of the confirmation and verification for businesses remains via SMS.
Interestingly enough, Instagram has none of this weird pricing stuff on their messaging API, despite being run by the same overlords
In a lot of those "poorer" markets WhatsApp isn't cheap-SMS, it's the actual commerce channel. People browse, ask and pay through it (India, Brazil, Indonesia, MENA), so even on thin per-message margins the volume and conversion are way higher than email/SMS ever hit there.
In richer markets it's still dominant for messaging across most of Europe and LatAm, RCS is still fragmented (Apple only just added it, carrier support is uneven), and email open rates are a fraction of WhatsApp's. So I'd push back a bit on "everyone settles on email/SMS."