I used to work for a company that had built up it's subscriber list over 10 years, we tried moving to a few other services but didn't get very far because they required actual physical proof that the user had opted-in to our mailing list, which we couldn't do (because nothing other than the email was ever logged). The only way to do it would have been to "resubscribe" all of the users to the new provider and hope they chose to opt in again. This likely would have lost us a significant percentage of our mailing list. There was no such requirement from Mailchimp, so you can guess who got our business.