There are many ways for big players to retain users even when they provide interoperability. Apple could‘ve been very smart about iMessage on Android, because it gives them a way to expose Android users to the Apple ecosystem by e.g. requiring the Android app to use an Apple ID. Combine that with keeping features like these exclusive to Apple devices:100% agree that a common standard would be better, but it also is very unlucky to emerge unless it is imposed by regulations because every messaging platform is basically a way to retain users in a proprietary ecosystem and profit from that user base one way or another, so there is no incentive for key players to go that route (actually things have been headed the opposite direction over time: old-timers will remember that back in the mid 2000s many major messaging platforms did implement XMPP and were interoperable, but all of them gradually retired it).
I also agree that proprietary gateways will make interoperability more difficult, but where we disagree is your assessment that it will be impossible/impractical to implement. Yes it will be a lot more work, but for someone who puts in that work, it will be very valuable as they will basically hold a universal key to all messaging platforms which is something most users want. One way it could evolve is for a few players to do all the hard development and maintenance work to access the various gateways, and bring to the market a middleware which has a simple API to abstract all that complexity for whomever wants to communicate with the gateways in a unified manner.
- Memoji recording (requires Apple hardware), show them as videos on Android
- iMessage App store (show fancy placeholders for apps / games on Android, allow stickers to be viewed but not added on Android)
- SharePlay on Apple devices (tease them with a chat message that says other Apple users started SharePlay)
- Apple Cash on iOS etc only
Stuff like basic texting, media attachments, replies, tapbacks etc could all be available on an Android app. Keep the bells and whistles to iOS only (stuff that requires the actual hardware / ecosystem).
To add to your API idea, someone potentially creates something like that but then again, it‘s still only be offered as a glorified wrapper app. You can‘t add interoperability for people not on your App, because WhatsApp and Telegram probably will never interop. Chats will be limited to one interoperability mode (e.g. only WhatsApp users and the custom app, because Telegram and WhatsApp wont relay messages between their clients. Your custom app is merely mimicking a half-assed WhatsApp client because it speaks „WhatsApp“ to the WhatsApp clients).
The beauty of interoperability should be that it doesn‘t matter what app I use, I can talk to people on other apps. Like Mail functions, which wont happen with this nonsense.