Your list doesn't really tell the story

Diaz/Lawler/Sylvia/Belfort/Arlovski/Sokoudju
All simply ran their course in the UFC or were not performing at the level they once were. Sobral was released for holding a choke to long. Lindland/Barnett/Shamrock have all had long time differences with Dana White. Shields has said multiple times that he would like to come to the UFC. Aleks isn't fighting for anyone b/c of allegedly having hepititis or a visa issue depending on who you ask. I don't think a UFC type system would work for boxing either though. UFC has 12-16 shows a year and they still have a hard time showcasing all of their fighters. With the number of boxers in the world it would be impossible to get all of the fights we wanted