Why pay for zoom and slack? Is something lacking from https://zoom.us/feature/messaging vis-à-vis slack ? Easy 'bot integration perhaps (slack certainly have a lot of integrations out of the box)?
They may also have a bit of a marketing problem. I was vaguely aware they had some chat functionality but didn't realize it was as feature competitive with Slack as it appears on that page.
I am a little confused about the pricing model for zoom;it looks like it's roughly one "host" pr team lead + extra money for "rooms"? And/or one host pr team member if you want full flexibility?