Email is the least-intrusive way for me to be contacted by people outside my organization. Some of them I do want to hear from and will be happy to transfer to a more synchronous or intrusive communication channel. Most I don't. Email is the best medium I have for screening those contacts.
Then there are marketing messages and announcements from platforms I use. I like having those pushed to me in such a way that I can easily ignore all but the 3% that are actually of interest.
And notifications. Yes, if a server is on fire, I want SMS or slack. But if someone's added a new ticket to our issue tracker or needs a pull request reviewed? Email is very effective for going through such notifications one at a time at one's leisure and dealing with each in succession.
Finally, there are the occasions where I want to share a big chunk of text with a bunch of people who may or may not be using the same platforms for other communications. It might be an announcement. It might be a list of questions for a vendor. Whatever it is. Email is very effective for that.
Now, what do you propose we replace all of those use cases with?