However, the fees they charge are very high. Many companies have been trying alternative cheaper products. The only reason they haven't switched is that Slack is still the best one in the market. Then the question is how big are its moats. In term of technology, there are nothing they have to defend newcomers. That is, there are no technological barriers. It is not hard to develop a similar product. In term of switch costs, it is basically none. Most chats in Slack, like in gchat, yahoo messager, etc. are time-sensitive. After one week, almost all the chats are useless. If a company is required to archive all the chats due to legal reasons, it is easy to do. Therefore, it is easy to switch to another messaging product, like it was easy to switch to Slack before.
I think the best outcome for Slack is to be acquired by a big company for a hefty sum, like Yammer was by Microsoft. If people can easily switch from Yammer to Slack, they will switch to next hot product with a blink.