I really feel sad for the close down of Mandrill, although we are not affected as we are also paid MailChimp user, so we can merge our accounts. However, we are considering moving away MailChimp due to this, just don't like the way they do business.
Have you looked into other alternate providers?
So, I'm trying SendGrid for transactional messages and sticking with Mailchimp for onboarding emails (a sequence of messages sent a few days apart to new contacts with consent of course).
Sendgrid was seamless to adopt. We'll see about deliverability.