I don't see an issue here. They made a useful app and probably would have at least a multi-million dollar business, eventually. As it happens, that was worth

I don't see an issue here. They made a useful app and probably would have at least a multi-million dollar business, eventually. As it happens, that was worth much more to Slack so they probably got a good immediate payday and stock in very promising company. For 2 years of work, I would say that's a great success.