Hmm, not really. All those apps fall into the trap of following a UX design where it's all about a group or person and the conversation within. IRC is channels

Hmm, not really. All those apps fall into the trap of following a UX design where it's all about a group or person and the conversation within. IRC is channels, Slack is channels, Riot seems to just basically be IRC, Discord is another take on the same genre. None of them give you what email gives you. With email there's not a person or group in sight in the UX, save for perhaps a 'from' entry in the thread list. It's all about info. I can start 100 conversations with the same person and have those threads organised into a timeline separately from each other. I can then also arrange them into arbitrary collections. I can possibly achieve something sort of like this by abusing groups or channels in these other apps, but then what? I just get a sidebar full of the things - what do I do with them? Can I search them and the info within from a global panel? Can I organise them into folders? Copy them to other accounts with a simple drag and drop? Email has quite a different focus than all those apps you mentioned which is probably why nothing has replaced it.