This concept of market saturation is a well known concept, however, you forgot one important part, the next generation. It is well known under marketeers that is it is hard to change habits. An 80 year old is not suddenly going to change toothpaste brand because they saw an amazing ad on television. However, a rebellious teenager/young adult will want to pick their own toothpaste brand, and are susceptible to marketing campaigns. Personally I have been using a different toothpaste brand because my teenage children want it, and they offered bulk discount, so not worth buying my usual brand in addition.

This is the same with Slack/Discord. If I can attract bright individual to work for me if I offer them Discord, I don't really mind switching to Discord.