No doubt it's going to be IM for most folks.
Now you have to download and run massive resource hogging apps like Team and Slack to simply communicate on 2 protocols.
And before that it was Snapchat! Met some young people who wanted to keep in touch with me a few years ago and had this funny conversation about email after me saying I didn't have Snapchat or Facebook or Instagram or WhatsApp. Oh, you can text me with SMS (that still works!) or email me!
e-mail is here to stay.
Nobody is saying email is going anywhere.
This is a good point and makes me wonder whether e.g. Matrix should incorporate something like a “prefers async” field that could be used by clients to treat messages more like mail.