Plenitude interferes with the internal logic of the stereotype. It says, actually, there are many kinds of micks, gays, jews, blacks, goths or Italians. And with this revelation the stereotype begins to come undone. If there are many kinds of micks then some of the assertions of the stereotype do not, cannot apply. And if some cannot apply, the larger stereotype loses some of its force and conviction.
Righteous indignation is harder to summon.
Prejudice more difficult to sustain.