I feel I can pretty authoritatively answer "Betcha not" here. </cofounder>
It's trivial to justify Slack's pricetag at any software company. Your entire team lives in it continuously and it becomes the nerve center of the company... for freaking pennies. It's ludicrous. If you add it up the non-Starfighter companies I run spend like $50k a year on SaaS/hosting/etc ("everything with an API attached") of which Slack is like ~1%, and probably the most underpriced-relative-to-value service which does not quote its pricing in picodollars.
IRC, mattermost, email, Gitlab, Wekan, Skype/Google Hangouts/BigBlueButton.... there are plenty of tools to communicate without spending loads of cash.
Seriously, Slack was the EASIEST things to justify as an expense when we proposed it to upper management earlier this year. And they are known to be very stingy.
I might hypothetically use an open source tool instead of Slack because it's not worth $2000 of productivity for 15 people per year.
If you have to pay for customers, then it's way too expensive.
I've seen a remote-based company put Slack to VERY good use, integrating it very tightly with their development process, I believe it definitely pays itself then.
If it's just a glorified IRC/XMPP, then definitely not.
The only upside of third-party SaaS is that you don't have to bother yourself to set things up and maintain them. Otherwise, I really don't see any limits that Slack can do but a self-hosted system can't.
What prevents you from using a same identity to log in to multiple self-hosted instances?
I think it's the same. As far as I get it - haven't used Slack much - with Slack you either have to use different email address to create account for each team (subdomain), or you have to be explicitly invited. Or maybe I got a wrong idea. Either way, while I'm not sure all self-hosted alternatives have options to consume external identities (be it third-party leased ones like Google account or self-originating ones), at least some have those. Which means the authentication is fairly transparent.
It may be nice to combine them in one place but it won't prevent anyone from using them.
This applies to Slack too - to best of my knowledge, one can't log in there, using, say, GitHub identity assertion as a credential.
But I'm not sure how Slack has anything to do with this. You're right. They're just a yet another platform with its own non-interoperable account system. Don't see how it's better than anything other, except when everyone is already there, which I find hard to believe. (But any other platform that knows how to consume external credentials is wins in this regard.)
Somebody made something, and is giving it out for free so that people can learn code, improve it or just use it. They haven't made the claim that this beats Slack today, but with a community of users and developers, it just might. Btw, it can be a success even if it doesn't fully outclass Slack.
Your comment is equally valid for the Linux kernel if it were 1991. Who would use a baby OS when they can get a well rounded commercial OS that crashes less often? Look at where we are now.
We had non-interoperable web services, and then businesses offering aggregation of those (like various social media aggregators, or even Slack) had appeared.
Now we have non-interoperable mobile apps.