They can answer whenever they like, but when I choose to read their reply I will have lost some of my train of thought or maybe lost interest in the topic. It doesn't allow for the same quality, or at least type, of discourse in my experience.

>IM is not a good medium

It certainly was a good medium - I had lots of great conversations on old IM services and so did countless others. See this related thread from the other day [1]

You could talk to many people at the same time, unlike a phone call. You could more easily do it at the same time as your homework, watching a film, or doing something else on the internet.

[1] https://news.ycombinator.com/item?id=19370281

You have to reinvent whole new protocols for those async conversations in the process, though, or misunderstandings or even outright failings in trust can ensue (as this article points out in a couple of examples.)

Any time you completely change a fundamental understanding of how something is done - especially something as critically foundational as communication - you end up having to build new rules quickly, or risk destroying either that foundation or trust in the new protocol.