That's an anticompetitive move. If you need to switch senders for some reason, the inspector is the only clean way to get an email's HTML into another ESP.
An email client like Thunderbird or Mail will save a copy of the email on your local hard drive, which will include the HTML. This isn't something I do regularly, but would be first first response if I needed to see the HTML of an email. Maybe Mailchimp has protections against this route too?
Even if you are a single organization user and leaving for good, you might do so gradually or perform test sends first. Speaking from experience again.