This means that the company will either need to keep raising money to buy out earlier investors that need to hit their fund target dates (Rather than raise it all for themselves), or they'll need to go public at some point, even if they don't need the money per se. Keep in mind that investors get lot's of sway via their board seats.
If you're in the business of losing money, this game of hot potato becomes harder and harder to play over time.
(Someone correct me if I'm wrong.)
Isn't this the literal definition of a Ponzi scheme? Isn't using new money to pay old investors technically illegal?
The line can be a little fuzzy, since some Ponzi schemers pretend to be an honest business and some real businesses end up being, functionally, a Ponzi scheme.
Slack simply chose not to do so.