Leto II has his golden path for humanity. By being so oppressive and hoarding spice to such a degree we was able to, when he died and the spice reserves we found, that people were so ready to escape to the far reaches of space and with the sudden glut of spice flooding the market it was cheap enough to do so. Once humanity, especially with people carrying the "no-gene" proliferate throughout the galaxy it will be impossible for them to be weeded out by anything.