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> they manage cross-device notification state better than... well Apple for one. That truly 'instant' messaging experience across devices is why they are superior to IRC and decentralized alternatives

I recently deployed workplace chat for ~100 people using XMPP. People use whatever clients they want (but we have a list of recommended clients, and in practice everyone is using one of those). With the modern extensions (SMACKS, carbons, MAM, push service integration) cross-device notifications are totally seamless. A bunch of the users are non-technical and they have had no trouble.

Slack has a lot of advantages over the likes of XMPP but I don't think technical superiority is one of them.