Here how I read this; "Since social login buttons aren't worth it, it's best to setup a login based on email and manage an email communication with your users instead of social ones. And by the way, Mailchimp may help you with your communication with your customers. Now remove those social buttons and come back to email".
Funnily, I agree with the conclusion and I'm using mailchimp extensively.
One key thing I think is to have the email as a username. I know this is generally done anyway these days but as someone who helps maintain a system where users have "usernames" separate to email addresses, there is a high support cost to this. I overhear a lot of support enquiries with "No, enter your username, not your email address! We've emailed you your username".
I think simpler is better and think using email instead of usernames should be what you go for first. However, I do think usernames are useful for public facing profiles in order to keep email addresses private. Another benefit is that usernames are usually shorter to type than email addresses (which is really noticeable on mobile...I should set up a shortcut for mine).
Most sites I've seen accept email or username in the login field. I'm pretty sure that was the default behavior for Django last time I looked into it (years ago).
I agree with your sentiment personally (email is guaranteed unique, for a start), but as the sysadmin of a company with a sixteen-letter-long domain... it does cause some problems, especially on mobile. Trying to type in a long domain on mobile compounds the problem of also having a long username, which is forced on you by most corporate email address formats, if you have a long name in meatspace.
Funnily, I agree with the conclusion and I'm using mailchimp extensively.