USA here. I used SMS/MMS for years before iMessage was ever even a thing, so it was natural to keep using the same app. I've never even had WhatsApp, FB Messenger, or any of the other messaging apps installed on my iPhones, and nobody I know uses any of them as their main messaging platform. Everybody I communicate with uses iMessage, or MMS if they're on Android phones. We can send text, photos, videos, web links, have group chats, etc. over the standard Messages app, and I couldn't care less what color somebody's bubble is or whether I can have different backgrounds/wallpapers.