> how many people buy iPhones just for being in their iMessage circles
This is news only in the US. Literally everywhere else in the world (where iPhones don't have a supermajority market share), third-party apps like WhatsApp, Telegram, Line, WeChat, Facebook Messenger, etc have been used for the past decade, with no problems whatsoever. My own WhatsApp account has been active for a decade, with chats going back exactly that far.
This is news only in the US. Literally everywhere else in the world (where iPhones don't have a supermajority market share), third-party apps like WhatsApp, Telegram, Line, WeChat, Facebook Messenger, etc have been used for the past decade, with no problems whatsoever. My own WhatsApp account has been active for a decade, with chats going back exactly that far.