You make a good point, one made stronger by the fact that Second Foundation is actually two somewhat unrelated novellas. They probably just decided to make three books because it made the marketing easier.

Hm. Two books would be pretty good, in fact. Book #1 to chronicle the rise of the First Foundation and the Traders as military and economic powers, and Book #2 to tell of the rise and fall of the Mule, and the Second Foundation's attempts to recover. Too bad the publishers like trilogies.