I agree with both
Sevanw and
widgeteer points on certain aspects.
But honestly, I see more and more users choosing to use alternate messengers over iMessage on their iPhones. But because most just leave iMessage on anyways, an iPhone to iPhone text will really be an iMessage. It's not actually because they consciously chose to use iMessage. That's the beauty of iMessage, but it also gives a false narrative of usage.
What I notice is many are using apps like Whatsapp and Telegram more than iMessage for deliberate chats(meaning not treated like texting), group chats, and picture/file sharing, instead of using iMessage.
One reason for this is the majority of iPhone users don't have a Mac or iPad. So they can't take advantage of using iMessage on anything else but their iPhone. But with other message apps, you can use them on a PC or Android Tablet. Another reason is other messaging apps are more robust.
What I'm really trying to say is .... I never looked at or treated iMessage as a messenger service, but more as a replacement of SMS texts with some messenger features. And that's pretty much how it was designed by Apple. But being that messenger apps are replacing texting as a style of communicating, I feel that iMessage is starting to become out of date.
For example: My 14 year old niece uses FB messenger with all her friends 99% of the time, but they all have iPhones. So why not use iMessage? I think it's because messenger apps are designed for continuous chatting, whereas iMessage is designed like SMS, which is more for sporadic texting.
End rambling