But also, 99% of the population is not technical enough to download such apps and use them. They will only use the default apps that came with their phone, whatever outdated protocol they happen to be utilizing. Why else do you think Facebook, for example, pays manufacturers to preload their crapware on new handsets? And even more to make it as difficult to remove as possible.
WhatsApp does not come pre-installed on most phones (for some reason it often isn't included even with Facebook's crapware). It is still the de facto chat app in India, Brazil and dozens of other Asian, South American and European countries.
As far as I know, SMS in India and Brazil is not free. This means that for majority of users switching to a data-based app is not so much as a choice, but a necessity. In the US texting via SMS has been free for several decades, so there's no reason for people to look for an alternative.
Exactly. It has reached critical mass to a point where if you're not on it, you're basically not communicating.
Also, I remember that cross-carrier calling was an issue, which is why most people would need to use dual-SIM phones. Not sure if it's the same for SMS.