Well, let me preface by saying I specifically avoided talking about vendor lock-in in my previous post for a reason. I only wanted to talk about why iPhone users perceive Android users negatively. I think this lock-in is generally bad.
I think there are actual, material reasons why Apple won't integrate with RCS until they're forced to, but also opening up iMessage is a very difficult proposition. Here is my speculation:
> refused to scale across the rest of the industry.
iMessage was not designed to be a decentralized chat protocol, RCS was. iMessage was not designed to work well with carrier UIs, RCS was. They are extremely different things. "scaling it" is much easier said than done.
> nobody is stopping them from opening the iMessage specification
iMessage is tied closely to apple accounts and centralized auth. It'd take a huge amount of engineering effort to open iMessage up, for zero benefit to Apple.
iMessage uses per-device-keys as a mechanism of spam control, and will blacklist physical devices. One of iMessage's key spam controls would be wrecked if it was "completely open". This means that android devices would have to be certified for imessage, to ensure that they have unique device keys, and that the manufacturer is actually operating in good faith.
They'd probably have to negotiate with individual device vendors for enrollment. How would you feel if iMessage was on Samsung devices only? Anyways, it's a huge amount of work that only yields negative strategic benefit.
Anyways,
Porting iMessage to Android is extremely non-trivial. RCS is a piece of shit (it really is, go look at the spec, the carriers put their hands all over it and it's rough).
Stop trusting major vendors, use signal or self host your chat.