Slack wasn't the first chat application either. The fact that it's a value add atop IRC is irrelevant, as Gmail is a value add atop IMAP.

I'm the OP, it doesn't really matter which was the first, my point was that we've had plenty of new "categories" of software. And in reality many that are just like Slack.

In some ways Google Apps for Business is a really good example of a sort of "new category" of communication software. You could plugin apps online, rather than have to host it your self.

A bit of googling shows he's called Slack a "brand new category of software" many times before, but I'm not clear what he thinks it is.

Yes and slack is not the first instant messaging program. /snarky quip about your age and IRC/

How many corporations use G suite for their email? How many roll their own email systems and use thunderbird?

To the average user, email is a proprietary non-federated system filled with tracking.

Gmail's a bad comparison because it never had to make money directly - it was just there to help Google build and maintain their search monopoly by acting as another data source and shutting out competitors. Slack, on the other hand, needs to make money because chat is their only product.