Nerfs don’t really affect popularity as much as you expected. If you want to check the actual popularity of a hero, quick play pick rate is a much bettter presentation because it is not affected by balance much. Even before sojourn was nerfed, her pickrate in quick kplay was still on the very lower end of dps.
Also, Your explaination of ball just makes no sense. Genji also requires quite amount of time to master, why is he so popular accross all ranks then? Or we can compare genji to echo, who is also a pretty hard hero. Why her pick rate has always been so low then? Clearly, many people just don’t want to play ball, simple as is