I have few friends with iphones here in Spain and of those all of them use WhatsApp. Not sure if they use iMessage with other Apple users but nobody ever uses it to SMS me (and if they did I would tell them to use something secure)
The main messaging apps in use here are WhatsApp (#1 by far), Telegram (used a lot for groups) and sometimes signal.
I've never had a friend suggest something else like Google, iMessage or Facebook/Instagram. I'm not on any of those anyway (even though I have an Android phone I don't use a Google account with it).
Note that I don't think many of us were using Kakao to talk with native Koreans, which would be a good reason for using it. Honestly, we didn't mix much. Nearly everybody on my contact list was an American.
Kakao is in many ways kind of the worst of chats - phone number-bound, unspec'd, closed, non-federated, unencrypted, known to be under surveillance, ad-infested. But on the other hand around 2010-2015 the UX was quite ahead of many others, and while Telegram & co have since pulled ahead it's still nicer than, say, WhatsApp or Signal.
A lot of people here have iPhones, but it's not because of some stupid blue bubble.
What's funny is that Americans will sometimes make fun of Japan because some people (mainly businesses, maybe some elderly people) still use FAX, but Americans are the ones who are all still using SMS.