I'm not sure I understand. Are you talking about the SMTP/LMTP format, or clients or what? If anything email lacks overhead - a proper thread meta data field that actually works across implantations (in-reply-to is a start, but not enough).
It's not like "xmpp overhead" made gräll unusable, is it?
I don't think that poster was talking about protocol innards, although kudos for you for having that knowledge, but perhaps they were, but in terms of user experience, slack/group chat is very low cost in terms of requiring any sort of mental pause before sending the message, specifically as a cost for the message itself. So much so, that a type of usage of verbal diarrhea can occur, so as to caution oneself against such usage, a little bit of thought before sending messages is always advised.
Specifically, if I can echo the primary differentiation, all you need to initiate a chat communication is a name and a message, that simplicity works a lot for people. I've used both e-mail and Slack, and a combination of both is great in my opinion, but the comparison can seen by an example message in my workflows (I don't use Slack anymore though):
Slack: 1) Find user/group by name lookup 2) Hi X, QUESTION_TEXT. 3) When done conversation, write outro
E-mail 1) Find user/group by name lookup, if user is not encoded in e-mail alias, query relevant databases 2) Write subject 3) Write intro, message, and outro
From my experience the reply from Slack 2) vs. E-mail 3) is much faster in the former case.
Which is much harder to both do in parallell and concurrently when busy with more than one snowflake problem at a time.
I'm sometimes shocked at how slowly things get done now, compared to when I worked a support desk in the early 2000s. But mostly I'm just a little sad at the overhead generated by chat.
[0] https://slack.com/help/articles/203457187-Customize-message-...
I think the crap is related to the amount of effort put into the message vs the medium. Making it easier/harder to send a message MIGHT change the quality, but I'd argue the decay of email over time was due to people failing to put in any effort DESPITE the minimum required.
I recall a coworker back in the late 90s talking about their usenet program, that made them click through a "Warning: What you are about to send will be read by thousands of people. Are you sure you want to send it?" message (paraphrased from memory). He found it daunting and therefore bad, I thought it was properly daunting.
Email was fine, nay, great, for people to have extended group discussions in, so long as everyone was willing to follow some rules. Then Outlook plus the Eternal September came along and now everyone is reinventing quoting repeatedly (and poorly). Branching is a whole thing (or mostly, not a thing when it needs to be a thing). And we don't even need to discuss what happens to HTML in emails.
Chat allows for fast questions, fast responses. This makes it harder to have lengthy or broader discussions in. Neither of those, however, says that chat has to ephemeral while email is lasting, which was the point I was responding to. If you want chat+history, you can't swap out chat with email and say "there, now you can have a history" - you just changed a LOT of things.