Actually the Slack I remember from when it first came out is more or less the same product they have now. I'm not really sure what all of those people were doing for all of those years.
Most people don't get to choose, in my case, it's our corporate IM system, so I have to use it whether I want to or not.
It's got a lot of enterprise features that companies like, like SSO integration, message retention policies, security certifications like HIPAA and FedRAMP, and more.
There are also a lot of pre-made apps for integrating with corporate apps like Jira, Gmail, Salesforce, etc.
It's fine, I haven't looked at a lot of other options, but it seems more usable than Google Hangouts (or messenger or whatever they call it now) or Microsoft Teams.
The big sells for Hangouts and Teams is that they have video. Slack doesn't so you have to find a video conference provider as well like Zoom. Also Teams integrates more easily with the existing Microsoft stack (office, outlook, etc.) that a ton of non-tech companies are on.
I think a lot of tech companies struggle to break into the non-tech world. Slack might be ubiquitous for SWEs but my SO who works in traffic engineering will probably never use it (they use Teams).
There was some technical evaluation done, but some features like retention policies and SSO were deemed requirements. Not sure what all of the criteria were or what all of the candidates were, it was a while ago.
You realize that there is a lot of stuff that goes on behind the scenes, right?
They need to scale their systems, maintain reliability, scale their processes, build internal tools, run experiments, work with 'legacy' code and within the existing architecture, etc.
It's not surprising product development has slowed down. In almost all cases it seems like the speed of product development is asymptotic.
Hiring too many people is still the number one thing that can kill your growing business. The communication costs alone eat a bunch of headcount so if you don't desperately need the people then you shouldn't hire them.
The problem generally starts when you hire a bunch of people without fixing broken processes first, these people then push back against fixing said processes, and the rot sets in.
Actually the Slack I remember from when it first came out is more or less the same product they have now. I'm not really sure what all of those people were doing for all of those years.