One argument that I could imagine (not sure that I'm supporting it) to prefer slack over mail: usage patterns for a shared email channel are far more individual than for a branded async medium like slack. With mail, some people will do elaborate filtering, others just dump everything in a single inbox. Some will use mail very close to real-time (perhaps even do all their mail hands-off through some Siri/Alexa/whatever voice interface?), others might maximise asynchronicity even to the point of hardcopies. A branded channel will be far more predictable in terms of client UI and usage patterns.