This kind of checks out for me. But also, there have been decent protocols around this for a long time, that many clients & servers have implemented. From Multi-device in the excellent Modern XMPP:*

> XEP-0280: Message Carbons - for "live" synchronization of conversations between online devices. XEP-0313: Message Archive Management - for "catch-up" of messages that were exchanged while a device was offline

https://docs.modernxmpp.org/client/protocol/

The XEP-0313 spec dates back to 2012 which is less old than I expected, and that's only the 0.1. So, very fair point.

More generally, the above complaint was about the development experience of XMPP. I feel somewhat like complaining about XMPP being failed is well tread from a why consumers didnt adopt it view, that negative sentiment abounds & everyone is more than happy to cast blame as to why. I've seen a lot less complaints about the development experience, and felt like maybe there was some novel fruitful grounds that a more developer-centric view might have been able to open up.