Having now spent a lot of time sending and analyzing email campaigns/triggered emails (I'm one of the co-founders of Klaviyo), we keep seeing that email is one of the best ways to turn analysis into action - not least because it gives you a complete feedback loop.
For example, say an Ecommerce store realizes most people only make 1 purchase and don't come back. With email, you can target exactly those people, but you also can quickly see if it worked (because you know who you sent it to, you can see if they actually made a purchase).
Retention is definitely really important, but I think what's at root here is that email is an ideal way to interact with users in a more targeted and personal way outside of when they proactively visit your website. The same feedback loop idea should (and will) apply to push notifications, texts, in-app messaging, etc as time goes on.
For example, say an Ecommerce store realizes most people only make 1 purchase and don't come back. With email, you can target exactly those people, but you also can quickly see if it worked (because you know who you sent it to, you can see if they actually made a purchase).
Retention is definitely really important, but I think what's at root here is that email is an ideal way to interact with users in a more targeted and personal way outside of when they proactively visit your website. The same feedback loop idea should (and will) apply to push notifications, texts, in-app messaging, etc as time goes on.