The problem with this, Leto was genuinely doing a great job. His Wormyness banned ursery and maintained a currency backed by a hard value*, had politicizing pseudohistorians burnt at the stake atop pyres of their own propaganda** and recognized that giving dissidents wives and families under the status quo meant they now had reason to support it***.
During the landsraad and later, Leto's empire, space travel was limited to a finite bubble of territory surrounding Arrakis, since a ship had to carry enough melange to sustain its Navigator both ways. After the Scattering, Ixian Navigation Machines and synthetic Melange produced in Axlotl Tanks broke this monopoly and allowed people to colonize everywhere. The idea being, that eventually new colonies would be being established at a rate faster than existing colonies could be conquered or destroyed, therefore, it'd be impossible for anyone to rule everyone.
*
**
***