A business that’s doing great can have numbers that look like this if they’re investing heavily in growth. In that case, they’re losing money deliberately—if they wanted to, they could optimize for profits and be making money, but at the cost of growing slower and presumably sacrificing greater profits in the future. If you’re a shareholder, you probably don’t want that.

I don’t know whether Slack is in this position, but simply looking at revenue and losses doesn’t give you enough information to judge how well they’re doing on its own. You need to look at things like CAC and growth rate.