"Why would you download an app for the basic functionality of a phone? Texting is an integral part of phones and I haven't met a person in the US without unlimited texting in years."
To your average iOS user they are one and the same, which is why US iPhone users don't generally download secondary chatting apps unless they have to.
Anecdotally, in my social group:
- Snap: withering on the vine.
- Messenger: generally declining with the declining popularity of Facebook in my age cohort
- Instagram: everyone has it, but messaging is definitely a second class part of the app and no one really uses it other than to send instagram memes
- GroupMe: I downloaded it once for one person, most of the people in the chat have iPhones anyways, and people don't really use it other than to contact the non-Apple users specifically.