For a while, many messengers actually shared underlying protocols (e.g. Google Talk & Facebook were both using XMPP at some point, and you could even cross-message).
Nowadays this is much harder. There's some exceptions (Telegram) with open client protocols, but I wouldn't wanna try and implement something like Discord, it'll be a never-ending tarpit.
They just don't want to fight people trying to build a full alternative client for Discord as a bunch of their paid-for stuff is just clien side javascript.
Whatsapp API works on the basis of conversations, the conversation has to be initiated by another party and only exists for 24hours from the last message from the other party. Sending messages unprompted is not possible unless it’s a templated message.
I can believe this exists to counter spam, and let’s not ignore the fact that WhatsApp messages through the API costs more per message than SMS.
Whatsapp is to the rest of the world what iMessage is to Americans.
And WhatsApp for "the rest of the world" is "free", about as free as Gmail and Facebook but monetarily free. It's hard to argue a monopoly when there is no money trading hands, and for business you are free to contact your customers via Email or SMS or whatever other form you would like, I can tell you there is benefits to using WhatsApp, our stats show much higher engagement and well we can actually get more information about message delivery than other platforms, you pay a premium for that and they gatekeep that because it has business benefit to do so.
If WhatsApp campaigns didn't get higher engagement than email or sms which is cheaper we wouldn't pay the premium for it, everyone who has WhatsApp can also receive SMS.
Does that help clarify why I'm arguing WhatsApp isn't a monopoly? It's kind of ranty, I apologise for that.
It allows me for example to avoid Instagram's crack app while still DMing with friends only available on there.
Except "Long term availability" ... I'd love to have my full chat archive under my own control but doesn't seem on the roadmap.