I don't see a chat program ever replacing email however. Instead, I see deeper integrations. Email, chat, sms, all working together. I think, eventually, all wanted communications will reach the right people over their preferred channels with data retrievable from any of the channels. Hopefully this can be done with open standards.
0: I work with Twilio SendGrid and am a principal engineer on the MTA team
1: https://litmus.com/blog/the-roi-for-email-marketing-the-good...
2: https://www.statista.com/statistics/271501/us-email-usage-re...
Pretty much everything email has modern chat can have too.