What is the impact of self-citations on your work when more citations comes from yourself, your students, colleagues, your intitution colleagues, but never outside?
If you interpret impact as changing the opinions of others so that they reuse your ideas or make different decisions in the future based on your work, then if you only get citations from yourself or your students and colleagues (who probably already have your opinion anyways), your impact is zero.
If there are no citations from outside, could you even make sure that someone else is reading your work? If the only purpose of the publications is to increase the number of papers, writing them is basically a waste of precious time.