As someone that used to studder in my youth, I can tell you that speaking plays a huge part in life. Whether fair or not it’s reality and just as problematic (if not moreso in many regards) than a lot of other disabilities because people judge your intelligence by how you express yourself. Speech impediments also make others incredibly uncomfortable.

Anyway, sounds like your dad was very successful because he was willing to change instead if taking offense. Having thicker skin in life is something I am always trying to improve upon.