WhatsApp has S-tier status here in Germany. If I had access to a proper API I would pay them per message, without them needing to make their UX worse. If anything, if I had to pay per message, I'd be incentivized not to send too many messages, keeping the distractions for the user at a minimum.
This is why they've been pretty draconian in banning users who work around the official apps and limits. Otherwise, to force their ads they would have to oust third-parties the way Reddit did.
MMS has terrible limitations, in both file size and media resolution.
RCS has replaced MMS as a protocol back in 2008 and it's only now gaining traction. Many carriers have shut down their RCS infrastructure half a decade ago, though, so they're not exactly jumping on the chance to turn it back on.
Whatsapp felt so responsive back in the day. I'd be pinging my family in real time halfway across the globe on mobile in 2009. For Free. That was a killer app...
