I don't believe this FUD. I think overal the market has moved from from charging for messages. I fully believe carriers would love to rip off everyone as much as they can, but I think carriers know customers would just use facebook messenger if they tried charging per-message.
Carriers charging too much is exactly why WhatsApp became so popular in so many countries. It was optimised for very little data usage, so was dramatically cheaper than SMS.
Don't forget you could also send photos with it, which is more than what can be said about MMS.
Funny enough, I think I was charged once for a MMS in the past 5 years. No idea how I managed to do it, but it seems to still exist and still be charged per message even on my unlimited everything else plan.
There's your problem right there. You may not remember being charged per message, but trust me, it was a thing.
The carriers would love to go back to 2 cents per sms, 5 per rcs "rich message" and 10 per encrypted "rich message".