I never get tired of railing against vanity metrics because people never seem to tire of following them.
The most noxious vanity metric in our world is email list size. There are more ways to inflate that number than you can count. They all produce a bad outcome, if you consider list fatigue and a declining brand as bad outcomes (I do).
Consider Nick Usborne’s timely rant on email gimmicks entitled “Why Strong Subject Lines Are Killing Your Email Marketing.” For sure, open rate is a sketchy metric in most cases. Usborne takes an extreme position, not without some merit, that the only metric really worth following is your reputation.
All good things have a beginning and an end. Your email list is not an inexhaustible resource.