Unless you know they never used Slack, so there are no expectations.

And your company wants to run OSS, so "has a guy" that already maintains the other pocketful of apps to company is using.

And since it is OSS, you can add Google OpenID auth if required.

And the slew of other features you want to add, but can't to Slack even if you knew about Slack.

Don't crap on the idea just because Slack is Slack and this isn't. Apply your "a month or so passes and now everyone is invested in the tool" scenario to any OSS alternative and your argument just comes off as fanboying.