Slack seems like they already had everything they needed. You can do video calls natively in Slack. You can store and transfer files. They just didn’t put the pieces together; everything was kept as a minor complement to chat.

That said, I’m not sure they could have competed with Teams in MS corporate deployments under any circumstances. If a company is already using Outlook, Exchange, AD, O365, Teams plugs in very nicely. But, seems like that still leaves a pretty big market.