> In contrast, email has been more successful thus far at resisting being walled off.

The point of e-mail was electronic mail: instantly sending text multimedia digitally. It's not necessarily been "walled off", but I think the wide spread adoption of things like Whatsapp, Facebook Messenger, Snapchat, MSN, ICQ and even SMS all happened because e-mail wasn't really convenient enough for instantly sending multimedia digitally at the time.

Now though, it would be an interesting experiment to force all chat/messaging apps to become fancy e-mail clients for e2e encrypted e-mails that they can't access.

The Web has fared better than e-mail IMO: it's far easier to find a website than it is to find an e-mail address, and people are far likely to go to something other than e-mail for the things e-mail can do.