A while ago I took over a small advertising agency to help a friend who got a new job and didn't want to close shop.
Long story short, they had WhatsApp groups for both internal communication and to talk to clients. It was a complete nightmare.
Clients would send audio messages at 1AM and expect an immediate response, important information was lost.
I introduced Slack and everybody in the agency loved and started using immediately. But the clients were all non-tech people and they just didn't want to use a new tool.
I honestly hope Slack succeeds in this battle. I'm a strong advocate and supporter.