It's about time someone made a unified messenger client that combines all these conversations and hides the detail of which service you are using to talk to friends.
Imagine you build it with a team of 5 software devs for each of linux, mac, windows, android, ios, web. You build it in Europe paying a generous salary of $80k. You need 1 server per million users (as whatsapp did pre-acquisition), hosted on say AWS. If you get 10% of the world to use it, the total running costs are 1.2 cents per user per year.
That can totally be paid for by sending each user 1 sponsored chat message per decade. That's a pretty acceptable level of advertising I'd say.
It's incompatible with VC's demanding hockey-stick revenue growth though...
Optimistic and dangerous. You will need at least a visionary person like Moxy willing to dedicate his/her life to the project. You need a master mind to get all the security architecture right. BTW You will need an operation team and multiple servers to make sure that everything runs stable. Just look the costs for running COVID-19 apps. Development and operation. Million of Euros. You can expect similar costs to run a service able to compete with WhatsApp.
I think the newer services are less interested in integrating or have weird and unique features that are harder to implement.
I agree though, it would be nice to be able to have Matrix, Discord, Telegram, Signal all on one program and with all their established features.
It was nice when I used it back in like 2014 because if WhatsApp went down I could just keep talking to the same person via Telegram (assuming they had both) and it'd all be in 1 joined conversation.
People are right, it doesn't solve the issue with Whatsapp, but it would mitigate some of the barrier caused by the network effect. I'm also sceptical of walled gardens, something usable in this domain would make this less of a problem when some drama rises over Telesignix, switching clients is a lot less of an ask than switching everything.
Suddenly multi-billion dollar chat apps with big network effects are mere service providers... And since Google has failed so many times at producing a popular chat app, I'm surprised they haven't gone down this route...