I've always thought that the "Myth of the Lone Genius" is a bit of a "motte-bailey" format argument, where an aggressive, undefendable stance is advertised, and then when challenged the person who is making the argument retreats back into a more believable stance. The bailey, in this case, is "no one person is truly responsible for progress or breakthroughs, and everyone's contribution matters", which is ridiculous, and the motte is "no man is an island, and all breakthroughs rely on the actions of others", which is obviously true.
I believe people that make the bailey form of the above argument want to believe that they're important, and want their contributions to be measured on the same scale that great people's accomplishments are measured on. If there's no such thing as the Lone Genius, then your lazy dissertation on a topic no one really cares about matters in the same way that the Bryce-Codd paper on data normalization matters, because "everyone's work is important".