Apple will use it's dominant position to create lock in like how they did with iMessage instead of cooperating with other platforms on a common standard.
Oder friends and family are surprised when they want to video call over Facetime and find it hard to believe other people's phones don't have Apple apps.
The app allows iPhone users to create an event. Anybody on any device or browser can RSVP. The event can be shared as a link. Making an event invite app that only works for users on one platform would be pointless.
Also - non-Apple users have been able to join FaceTime calls via. A link for several years.
It's not the color itself that's the problem, it's that having one green user means the entire conversation falls back to SMS and thus photos, videos, etc are all degraded and you can't do more rich messaging things like reactions. This is changing with RCS but it is in Apple's interest to make it a social change rather than just a technological limitation.
> it is in Apple's interest to make it a social change rather than just a technological limitation.
It is a technical requirement? How would non-iMessage users respond to the whole group including the ones on iMessage?
When you sit for 5min and think about the whole flow across a bunch of message exchanges every other way there's really no other technical solution than downgrading the whole conversation to SMS/RCS.
There is no authoritative mapping from an account to a single service (e.g. my email address as an Apple account vs a Google accounts vs a WhatsApp account), which also means that if all three of these services say they have an account for me and advertise a public key, there is no way to know that account or public key are authoritative. Google's implementation requires you to use both their client and their hosted service, meaning it almost certainly assumes that all E2E keys can be resolved authoritatively from a single source (Google's table).
You instead need a way to look up accounts in a secure and auditable way across multiple authoritative services, like the IETF Key Transparency work (that isn't complete yet).
It is also important to realize that Apple's support for alternative messaging systems besides iMessage is to meet carrier requirements, not user requirements. Apple's slow uptake on RCS AFAIK was because carriers themselves didn't care, until governments began to regulate it needed to be supported on handsets. The carrier RCS support almost universally is because Google wanted it for Android, which is also why Google's RCS hosted service is by far the most deployed by carriers.
The GSMA needs to define those carrier requirements for E2E RCS, and Apple has stated publicly they are working with them on that.