Slightly ungracious of me to start with a point of information, but The Five Faces was two years before the twentieth anniversary, in November 1981 - but yes, it was chosen because it was the only four-parter available at the time, where now it would only be one of two... I agree with your assessment. There's little to get hold of, the Krotons are rather limited for such a brilliant species, and the politics of the Gonds is run-of-the-mill, despite the presence of Philip Madoc.