If you’re my customer and a Gmail user and I send you order details, financial/credit info, any number of things I might know about you that Google might not know already, Google learns that about you from me.
Instead, I’m going to send you an email that prompts you to come back to my site or app to get your tracking number or flight info or whatever.
It might not be some other communication service that kills email, but rather apps and push notifications trying to outmaneuver Google.