These are great points, though I disagree with all of them as reasons to prefer email over chat.
> 1) length -- most emails are significantly longer than any single chat message
Agreed. The beauty of a chat, rather than an email thread, is that each message contains (or should) a single point. Which means the chat thread is more of a sequence of the following kinds of responses: Yes, and; Yes, but; No, and further; No, but. This makes for a better conversation, and makes it easier to process. How many times have you asked two questions in an email, only to have someone respond to just the last one?
>2) context -- email threads are saved (and indexed) in the context of their thread.
>With chat, you really don't get ether, so it's significantly more difficult. You can never really know what a message is a reply to...
This is a matter of implementation, not technical capability. A decent context is achieved through proper use of tags and chronological ordering. Better threading can be achieved through a protocol implementation of a reply-id capability, which most chat systems have (XMPP, Discord, etc.), coupled with GUI presentation of messages in a threaded view.
>...Responses can come fast or slow...
True of any asynchronous channel, email or chat.
>Individual channels can have multiple conversations happening, if not simultaneously, then at least over time.
I see this as a feature, not a bug. With tagging a chat message can be part of several different conversations at the same time, even across channels. Which conversation you are presented with depends on which tag you are viewing.
>I mean, that's the difficulty with any system... getting people to use it. If your team can be disciplined about the tags, then I'm glad that works for you. I not sure it would work well in many other contexts.
In my experience, this is often the biggest hurdle in implementing any revolutionary system (as opposed to evolutionary) that makes a big change in how people do things. For messaging, there are systems with a reminder feature similar to most email client's behavior when there's no subject. For instance, "This message has no tags. Messages are required to have at least one tag, and should have between 2 and 7".
You need management to force the issue though. An approach I've used in the past is to find a competitor or two using the technology, and use business/financial FOMO as a motivator.
Also, if you do a typo in an email..it's there forever. A typo in a chat can be fixed in many systems.
In a desire to be fair, I will add two reasons to use email:
1) If you are doing CYA. Email can't be changed, and is routed through a number of servers, possibly leaving log fingerprints. Email also has bcc. Anything that is important you can bcc to a second friendly party to prove you sent it, or to a personal email account (do this with trade secrets, or heaven forbid - classified - and you should get what you deserve, and I don't mean in a good way). If you want a paper trail, don't use chat.
2) If you are sending a document. Most businesses have virus scanners built into their email server software. A document is typically sent, retrieved, and saved somewhere. Though, if a document will be referenced often by many, neither chat nor email are best. Use a wiki, or something similar.