Maybe, so far I've been in a few situations where we didn't go with Slack...only to go back to Slack. Part of the lock-in is their brilliant multi-org client. I can easily go back-and-forth between my company, a contract's IM, an alumni group of people in my last job and a group of local artists that like to talk to each other. Hell I've thought about luring most of my close friends from Facebook onto Slack.

Their API is also quite good IMO, which provides a sort of lock-in, though not a huge one.