Gmail was not the first email program (which is what the OP asked about). Gmail came several decades after email was invented and already in widespread use. You may or may not realize this fact (depending on your age).
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.
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.
Sorry, I don't mean that as snark. Many people born after 2000 have known nothing but Gmail as email and are surprised to learn that other email systems/providers exist.
That's nothing unique to age. I regularly talk to people born before 1960 (and 1970, and 1980....) who don't know that email is separate from it being on a website (about the only difference is that they know of Yahoo Mail, Hotmail, etc)
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.