I'd wager that Leto's plan was more effective than any meek plan of colonisation would have been. Leto burned a memory of tyranny into the human spirit – and it is important to realize that memories function differently in the Duneverse, given the fact that they are retained across generations. This isn't limited merely to prophets and Bene Gesserit, who have conscious access to those memories: even other people have ancestral memories, which influence them unconsciously:
Memory has a curious meaning to me, a meaning I have hoped others might share. It continually astonishes me how people hide from their ancestral memories, shielding themselves behind a thick barrier of mythos. (...) We carry all of our ancestry forward like a living wave, all of the hopes and joys and griefs, the agonies and the exultations of our past. Nothing within those memories remains completely without meaning or influence, not as long as there is a humankind somewhere.
As a people, you react against threats to innocence and the peril of the helpless young. Unexplained sounds, visions and smells raise the hackles you have forgotten you possess. (...) This is system-feedback at its most primitive level. Your cells remember.
Leto's lesson is very simple: it is a lesson that teaches mankind independence, a lesson they will never forget. They are ever to flee central government and continue along their own paths, instilling a desire for freedom that would set in motion Scattering after Scattering:
When I set out to lead humankind along my Golden Path, I promised them a lesson their bones would remember. I know a profound pattern which humans deny with their words even while their actions affirm it. They say they seek security and quiet, the condition they call peace. Even as they speak, they create the seeds of turmoil and violence. (...) I give them enduring eons of enforced tranquility which plods on and on despite their every effort to escape into chaos. Believe me, the memory of Leto's peace shall abide with them forever. They will seek their quiet security thereafter only with extreme caution and steadfast preparation.
It is galling to be held in place against your will. This is why I teach about tyranny in the best possible way - by example. Even though you read these words after a passage of eons, my tyranny will not be forgotten. My Golden Path assures this. Knowing my message, I expect you to be exceedingly careful about the powers you delegate to any government.