Those 50 largest customers are unlikely to move away within the next two years, and neither are most businesses. I can see someone worry about growth, but they're not going away in 2 years.

Two years ago, those 50 customers weren't using Slack at all. It will be just as easy for them to leave.

Especially when Microsoft just tosses Teams in for free in their next Office 365 renewal.

My company just switched to Teams (from Sametime). We had evaluated Slack, Teams, and Cisco/Webex Teams. To me Teams is a pretty crap chat tool, but it's integration with WIndows (and the macOS client isn't shabby either) is what made it a clear winner. We can chat, do voice, share files, collaboratively edit files, share desktops, all in one app. My only complaint is that the emoticons are typical Microsoft style and should be updated to something better. But this might be due to most Windows desktops not using a high DPI. On a Retina screen they look amateurish.

In large enterprises, it's hard to break into the enterprise but once your application is used you are in there for the long run until unless you have an outage while they have a major incident.

With Slack, if you have a few integrations setup or a bot or 2 configured, it's hard to move. Also the alternatives are terrible.

Why would they?

A free alternative?

You guys really think that enterprise-tier Fortune 500's are going to switch from Slack to a free alternative? Why would they? The money they spend on Slack isn't even a rounding error in their budgets, there's no way that they would want to cut that out and instead have their internal staff try to support some free service instead

Discord has essentially the same app but better voice/call support and other various features. But they seem focused on the gamer market.

When they realize they are tired of the distraction and didn't really need it in the first place.

Your sentiment is valid, but you need to quadruple the timescale.