Since vanity metrics can give you false confidence when they’re performing well, they have the ability to do the opposite. When they’re not performing you start doubting the worth of the project, or worse yourself. You could think you’re not having an impact, which might be true. But if they’re misaligned metrics are harmful, it might not even matter.
One example is if you’re writing to learn, like learning in the open, having low or no page impressions might demotivate you and make you stop learning in the open. Even thought you’re actually learning and building an understanding.