When Slack came up, some of our development teams picked up using it, because it was free and didn't have to be installed and because of "it was that hot new thing from Silicon Valley that all devs use now". Most people loved it. I've always been in the "we try to keep our other infrastructure mostly in-house for control/privacy/anti-lock-in reasons, so why add a dependency to an external service and push internal documents onto it while we don't even pay for that thing?" camp and therefore flat-out refused to use Slack until the issue of internal instant messaging got enough attention to be discussed on a company-wide level. It was agreed upon the need for such a service, but it was also recognized that the way in which Slack was used to serve that need was not sustainable and far from rational, so alternatives were discussed and we eventually settled on trying out Mattermost.

Within weeks, all teams using Slack had moved over to Mattermost, and even most of the people who never wanted to use Slack for various reasons, including me, were okay with giving Mattermost a try. Most kept using it, so it has become a widely accepted part of our internal infrastructure now.

Others mentioned Mattermost, Discord, but really, there are so many viable options out there. See this list of 25+ alternatives for example: https://fleep.io/blog/best-slack-alternatives/

From cloud-based to self-hosted, open-source and commercial... As long as they integrate with the tools devs use, there will be devs who use it.