I profess I am not a particularly tech-savvy user, and I am having a hard time envisioning how that would work.
For example, how might this work between say, WhatsApp and Telegram (just the two messaging apps I am familiar with). Telegram stores their messages in the cloud, so the advantage is that I can download telegram on another device and log in and all my past messages are all there. I also get a native desktop client without any of the relaying nonsense.
Conversely, WhatsApp pushes messages to the recipient's device. I guess it's better security that way? But it also means that if I restore an earlier cloud backup onto another device, I miss out on say, all the messages that took place after said backup.
They also have somewhat differing features like different animated stickers, and different editing timeframes (telegram lets you edit messages of up to 2 hours ago, WhatsApp has a cap of just 15 min). But at the end of the day, each has their pros and cons, and I get 2 sufficiently differentiated products because each is allowed to operate on their own terms and not have to worry about compatibility.
How are these two devices supposed to work interchangeably with each other, while preserving what makes each of them unique?
How is the backend even supposed to work, and who would be in charge of overseeing that exactly? Reminds me of that comic where it just means we end up with one more messaging standard in a bid to unify them all.