What the interviews invariably show is that a chat client is not simple. It's extremely complicated, especially when you want it to work at scale. If you think it's a "simple" app, go ahead, try to make one - even on a whiteboard.
I can completely understand why instant messaging can get you $700 million in revenue. As for the losses -- they are invested in the future of the company and I think are justified.
"How hard can it be?" is something we ask about "simple" stuff everyday, and the answer if you dig deeper is almost always "Much harder than you think."
A large plurality of it is sales & marketing. The next biggest chunk is R&D.
S&M = $233,191 for 2019
More than half their revenue is spent acquiring new customers...which, with a high likelihood, will net revenue over a N+1 year timeframe. This is an investors wet dream... I pay $1 now and I only need $.15 to operate that $1 every year for the next 7 years...that's a helluva return.
In other words...you can turn off the S&M tap and these companies could be profitable almost overnight.
Slack doesn't have that much lock-in and a lot of people have their sights on that market -- "Cheaper than Slack and bundled with Office365/G Suite" is an extremely tempting offer.
Their API is also quite good IMO, which provides a sort of lock-in, though not a huge one.
While not quite as fully-featured as Slack, Keybase offers this. I'm a member of multiple orgs and seamlessly change between conversations (both group and 1-on-1).
Yes, they have a boatload of fat to trim. They can pull an Etsy move but is that not already baked into their valuation? I, frankly, don't care enough to look as I won't be investing. I just cannot believe their VC investors let them get that bloated - it's probably why they're pushing for an exit so fast. I also am not convinced Slack is very sticky. We switched to Mattermost last week and miss nothing about Slack.
Moreover, as I said in another comment: If there is all this money left to be made, why are their investors pushing them to go public? They should be riding that gravy train privately and cashing out a few years from now for 10x more...
This isn't a consumer product. They're not doing magazine and television ads. Why is S&M so expensive?