In my case, for one, the engineering orgs were generally not much larger than 200 people. For another, the "To:" person(s) usually responded, but everyone else got cc'd with the list and then the list could follow. And, for me, finally: I find discoverability in a knowledge-base-way to be sub-optimal compared to an indexed (and threaded!) email list. Kind of like how Linux devs organize themselves.

And, maybe, finally, I'm old and have been using email since 1983 and it fits my workflow (or rather: my workflow adapted to email?). The get-to-it-or-it-is-gone thing with #slack bothers me, which seems great for asking people what they want me to bring back for lunch, but not for much else besides asking who broke the build.

Again - that's my take. I understand that other people don't prefer long-form debates/discussions by email and want an IRC-like answer to their immediate question.