Well, in case anyone else reads this: I mean that if you click the huge “message” button at the top of a contact’s page, that opens iMessage, and there’s nothing I can do to change that if I don’t want to be using iMessage.

For contrast, Android lets users use third party texting apps, remove the default messaging app, have all “message”-oriented actions open the app of your choice, etc. Apple, I claim, does not support this because it means that every iPhone user is also an iMessage user. But iMessage is a social network (a la WhatsApp), and a separate product.