I'm afraid that at this point Slack has too strong a network effect to be replaced with something better except in niche circles.
In addition the communication "niche" is MASSIVE. Communicate, as humans it's what we do. There's a reason why FB paid soooo much for WhatsApp, yes?
If add-on can make this product something between Slack and (the shitty implement of) FB groups, there's plenty of room for such a thing.
Maybe fax machines make a better technical comparison. Email replaced their intra-company use in a few years. What kept them in use is that they are customer facing. Similarly, Skype and such replaced copper telephone lines for internal conference calls and in many businesses cell phones replaced landlines for intracompany communication and the phone system exists because it is customer facing.