I agree with you that terminating the rep is not interesting, and I think you're mistaken if you feel like that's what anyone thinks will solve this problem.
Actions SendGrid could take:
* Make it impossible for their front-line support staff to change the email address on file. If you want that -- which should be extremely rare! -- you talk to a high-level manager who is competent at authenticating you.
* Send the email that says "hey, we're going to change your email address now" with a lead time to allow for the possibility that, even after your authentication, you've been conned.
* Make a phone call to the phone number on record, too.
You ask what's gained by faulting SendGrid, because you take it as a given that they will make these changes. But that's not how blame works. The blame serves a function of ensuring those changes by holding them accountable for their current problems.