But iMessage is already open? You can send an SMS to any number and it shows in iMessage, completely interoperable through that standard protocol.
Whatsapp on the other hand does not show SMS messages (Which is a design choice that makes sense from a security perspective I guess, not saying it's wrong.)
> But iMessage is already open?
How do you send/receive messages from a Windows system? My guess is that you think iMessage is SMS-only.
> You can send an SMS to any number
Can you send a photo?
You're confusing two different things, though I don't blame you for it, as it is confusing. "iMessage" is the OTT E2E-encrypted chat protocol. "Messages" note the lack of the leading "i" and trailing "s") is an iOS app that lets you send and receive messages using both the iMessage and SMS/MMS/RCS protocols.
iMessage is not open, and Apple fights efforts by other companies (e.g. Beeper) to interoperate with it.