Republics have a longer life and enjoy better fortune than principalities, because they can profit by their greater internal diversity. They are the better able to meet emergencies.
Where some people are very wealthy and others have nothing, the result will be either extreme democracy or absolute oligarchy, or despotism will come from either of those excesses.
The case for trade is not just monetary, but moral. Economic freedom creates habits of liberty. And habits of liberty create expectations of democracy.