I think my main point is "it's not true that your only options are Be a Hero or Be Insignificant, there's a third option." Because if it's presented as a dichotomy, I think many sidekick-oriented people would go for being insignificant–so the impact they could theoretically have as a hero is moot.

It's important to remember that what you want to focus on is expected impact. Of course the sidekick can't have as much impact as the hero could have, but if there aren't enough people who want to be sidekicks relative to heroes and you have a comparative advantage as a sidekick, you probably have higher expected impact as the sidekick to your pick of hero than becoming a mediocre hero yourself.