The fundamental problem with replacing Slack is network effects. Your coworkers and customers already use Slack. It works well enough. You can choose to switch your company away, maybe, but what do you do when vendors want to connect over Slack? Imagine if email was owned by a company? Edit: W̶e̶ ̶r̶e̶a̶l̶l̶y̶ ̶n̶e̶e̶d̶ ̶a̶n̶ ̶o̶p̶e̶n̶ ̶p̶r̶o̶t̶o̶c̶o̶l̶ ̶t̶o̶ ̶b̶u̶i̶l̶d̶ ̶o̶n̶.̶ We really need an open protocol to win…
Like XMPP? Or, perhaps the asynch chat thing is a distraction and we need something asynchronous that's well proven. Like... email? Slack should never have been a thing IMHO. I remember first using it at a startup I was CTO of at the behest of the CEO ("everyone is using it"), back in around 2013. Instantly hated it. Just wish we could go back to good old email, TBH.
Real-time chat is, in fact, useful, and a separate product from email. The fact that you don't want to use it does not change the fact that others do.
I use Zulip and Signal extensively, and I use email occasionally, and none of them fully replace the use cases of the others.