Oh Symphony was nothing like Slack. It was meant to be the Bloomberg IB killer[1]. You see, banks weren't happy paying 20k per user for chat. Turns out that the network effect is a thing and banks suck at building products, all Bloomberg had to do was sell Chat as a product and that was it [2].
[1] https://techcrunch.com/2017/05/16/symphony-a-messaging-app-b...
[2] https://www.ft.com/content/f16d73ee-a910-11e7-ab55-27219df83...